Bandgap grading in thin-film devices via solid group iiia particles

ABSTRACT

Methods and devices are provided for forming thin-films from solid group IIIA-based particles. In one embodiment, a method is provided for bandgap grading in a thin-film device using such particles. The method may be comprised of providing a bandgap grading material comprising of an alloy having: a) a IIIA material and b) a group IA-based material, wherein the alloy has a higher melting temperature than a melting temperature of the IIIA material in elemental form. A precursor material may be deposited on a substrate to form a precursor layer. The precursor material comprising group IB, IIIA, and/or VIA based particles. The bandgap grading material of the alloy may be deposited after depositing the precursor material. The alloy in the grading material may react after the precursor layer has begun to sinter and thus maintains a higher concentration of IIIA material in a portion of the compound film that forms above a portion that sinters first.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/762,060 Jun. 12, 2007, which claims the benefit of priority to U.S.Provisional Application Ser. No. 60/804,565 filed Jun. 12, 2006, Ser.No. 60/804,566 filed Jun. 12, 2006, Ser. No. 60/804,567 filed Jun. 12,2006, Ser. No. 60/804,569 filed Jun. 12, 2006, Ser. No. 60/804,649 filedJun. 13, 2006, and Ser. No. 60/804,647 filed Jun. 13, 2006. Allapplications are fully incorporated herein by reference for allpurposes.

FIELD OF THE INVENTION

This invention relates generally to semiconductor films, and morespecifically, to semiconductor films containing a group IB-IIIA-VIAcompound and formed in part from solid group IIIA-based materials.

BACKGROUND OF THE INVENTION

Solar cells and solar modules convert sunlight into electricity. Theseelectronic devices have been traditionally fabricated using silicon (Si)as a light-absorbing, semiconducting material in a relatively expensiveproduction process. To make solar cells more economically viable, solarcell device architectures have been developed that can inexpensivelymake use of thin-film, light-absorbing semiconductor materials such ascopper-indium-gallium-sulfo-di-selenide, Cu(In, Ga)(S, Se)₂, also termedCI(G)S(S). This class of solar cells typically has a p-type absorberlayer sandwiched between a back electrode layer and an n-type junctionpartner layer. The back electrode layer is often Mo, while the junctionpartner is often CdS. A transparent conductive oxide (TCO) such as zincoxide (ZnO_(x)) doped with aluminum is formed on the junction partnerlayer and is typically used as a transparent electrode. CIS-based solarcells have been demonstrated to have power conversion efficienciesexceeding 19%.

A central challenge in cost-effectively constructing a large-areacopper-indium-gallium-di-selenide (CIGS) based solar cell or module isthat the elements of the CIGS layer must be within a narrowstoichiometric ratio on nano-, meso-, and macroscopic length scale inall three dimensions in order for the resulting cell or module to behighly efficient. Achieving precise stoichiometric composition overrelatively large substrate areas is, however, difficult usingtraditional vacuum-based deposition processes. For example, it isdifficult to deposit compounds and/or alloys containing more than oneelement by sputtering or evaporation. Both techniques rely on depositionapproaches that are limited to line-of-sight and limited-area sources,tending to result in poor surface coverage. Line-of-sight trajectoriesand limited-area sources can result in non-uniform three-dimensionaldistribution of the elements in all three dimensions and/or poorfilm-thickness uniformity over large areas. These non-uniformities canoccur over the nano-, meso-, and/or macroscopic scales. Suchnon-uniformity also alters the local stoichiometric ratios of theabsorber layer, decreasing the potential power conversion efficiency ofthe complete cell or module.

Alternatives to traditional vacuum-based deposition techniques have beendeveloped. In particular, production of solar cells on flexiblesubstrates using non-vacuum, semiconductor printing technologiesprovides a highly cost-efficient alternative to conventionalvacuum-deposited solar cells. For example, T. Arita and coworkers [20thIEEE PV Specialists Conference, 1988, page 1650] described a non-vacuum,screen printing technique that involved mixing and milling pure Cu, Inand Se powders in the compositional ratio of 1:1:2 and forming a screenprintable paste, screen printing the paste on a substrate, and sinteringthis film to form the compound layer. They reported that although theyhad started with elemental Cu, In and Se powders, after the milling stepthe paste contained the Cu—In—Se₂ phase. However, solar cells fabricatedfrom the sintered layers had very low efficiencies because thestructural and electronic quality of these absorbers was poor.

Screen-printed Cu—In—Se₂ deposited in a thin-film was also reported byA. Vervaet et al. [9th European Communities PV Solar Energy Conference,1989, page 480], where a micron-sized Cu—In—Se₂ powder was used alongwith micron-sized Se powder to prepare a screen printable paste. Layersformed by non-vacuum, screen printing were sintered at high temperature.A difficulty in this approach was finding an appropriate fluxing agentfor dense Cu—In—Se₂ film formation. Even though solar cells made in thismanner had poor conversion efficiencies, the use of printing and othernon-vacuum techniques to create solar cells remains promising.

It should be understood that some precursor materials used in non-vacuummanufacturing of thin-films suitable for semiconductor devices may be inliquid form, with these precursor materials serving as source materialfor the thin-film, whereas most other precursor materials in the ink arein solid form and desirably so, this in contrast to materials added tothe ink to allow for reliable, fast and uniform deposition, likesolvents and organic additives. These solvents and organic additives aretypically unwanted in the final thin-film and require facile removalduring or after the deposition process. Unfortunately, sometimes thesepreferably solid components can become liquid at the handling and/orparticle size reduction temperatures typically associated withnon-vacuum techniques for solar cell production. This may be adisadvantageous feature as premature and/or undesired liquification orcoalescence increases the difficulty in handling these materials duringprocessing, during ink storage, and may require more involvedtechniques. For example, elemental gallium is a liquid above 30° C.,which is very close to room temperature and below the processingtemperature associated with deposition and/or ink preparation. It mayalso be disadvantageous during processing since the liquid form maychange the kinetics of the conversion of the particulate layer to thefinal semiconductor film. For example, if too much liquid is present ator near the onset of a reaction, liquid may group together at certainareas and not be evenly distributed throughout the reaction area. Thiscan result in thickness non-uniformity and/or lateral compositionnon-uniformity. Furthermore, if material in liquid form leaches out froman alloy or compound containing that material, this may change the localstoichiometry at the start of the reaction resulting in differentcompound(s) in the final thin-film if the leaching occurs prior to orduring processing of the materials.

For example for the preferably solid components, liquid form might bepresent and undesirable before/during the synthesis of the particles.Such components in liquid form increases the difficulty in controllingand maintaining the particle (droplet) size during ink preparation andsolution deposition. In one example, elemental gallium used in thin-filmsolar cell production is a liquid above 30° C., which is very close toroom temperature and below the processing temperature typically usedduring ink deposition. Lowering the processing temperature far below themelting point of gallium complicates the ink preparation and solutiondeposition. Additionally, difficulty in controlling the particle(droplet) size during deposition complicates controlling and maintainingthe target thickness uniformity of the resulting film on micro-, andmacroscopic length scales.

Additionally for the preferably solid components, liquid might bepresent and undesirable when annealing the coatings of ink. It may alsobe disadvantageous during single and/or multi-step conversion of thesolution-deposited coating or layer into the resulting semiconductorfilm since the premature presence of liquid may change the kinetics ofthe reactions involved and therefore the quality and uniformity of thesemiconductor film. For example, if too much liquid is present at ornear the onset of a reaction, liquid may dewet from the surface and ballup resulting in a non-uniform material distribution throughout thelayer, both in thickness and composition.

Due to the aforementioned issues, there are significant opportunitiesfor improving non-vacuum CIGS manufacturing processes. Improvements maybe made to increase the throughput of existing CIGS manufacturingprocess and decrease the cost associated with CIGS based solar devices.The decreased cost and increased production throughput should increasemarket penetration and commercial adoption of such products.

SUMMARY OF THE INVENTION

Embodiments of the present invention address at least some of thedrawbacks set forth above. The present invention provides for the use ofsolid particles in the formation of high quality precursor layers whichare processed into dense films, semiconductor films, and/orsemiconductor dense films. The resulting films may be useful in avariety of industries and applications, including but not limited to,the manufacture of photovoltaic devices and solar cells. Morespecifically, the present invention has particular application in theformation of precursor layers for thin film solar cells. The presentinvention provides for more efficient and simplified creation of adispersion, and the resulting coating thereof. It should be understoodthat this invention is generally applicable to any processes involvingthe deposition of a material from dispersion. At least some of these andother objectives described herein will be met by various embodiments ofthe present invention.

In one embodiment of the present invention, a method is describedcomprising of providing a first material comprising an alloy of: a) agroup IIIA-based material and b) at least one other material. Thematerial may be included in an amount sufficient so that no liquid phaseof the alloy is present within the first material in a temperature rangebetween room temperature and a deposition or pre-deposition temperaturehigher than room temperature, wherein the group IIIA-based material isotherwise liquid in that temperature range. The other material may be agroup IA material. A precursor material may be formulated comprising: a)particles of the first material and b) particles containing at least oneelement from the group consisting of: group IB, IIIA, VIA element,alloys containing any of the foregoing elements, or combinationsthereof. The temperature range described above may be between about 20°C. and about 200° C. It should be understood that the alloy may have ahigher melting temperature than a melting temperature of the IIIA-basedmaterial in elemental form.

For any of the embodiments described herein, the following may alsoapply. The group IA-based material may be a Na-based material. The groupIA-based material may be comprised of NaF. The group IA-based materialmay contain an element chosen from the group consisting of: sodium (Na),potassium (K), lithium (Li), Rubidium (Rb), Cesium (Cs), Francium (Fr),an alloy containing any of the foregoing, or combinations thereof. Thegroup IA-based material may be comprised of an elemental material. Thegroup IA-based material may be comprised of a binary alloy. The groupIA-based material may be comprised of a multinary alloy. The groupIIIA-based material of the first material may be Indium. The groupIIIA-based material of the first material may be Gallium. The alloy maybe a binary alloy and/or a multinary alloy. The alloy may be comprisedof a Ga—Na based alloy. The alloy may be Ga₄Na and/or Ga₂₉Na₃₂.Optionally, the alloy contains at least about 0.6 weight percent Na. Thealloy may contain at least about 8 weight percent Na. The alloy maycontain at least about 11 weight percent Na. The alloy may include anIn—Na based alloy. The alloy may be comprised of In₈Na₅. The precursormaterial may contain particles comprised of Cu-based particles. Theprecursor material may contain particles comprised of Cu-based alloyparticles. The precursor material may contain particles comprised ofCu-IIIA based alloy particles. The precursor material may containparticles comprised of Cu-VIA based alloy particles. Optionally, theparticles may be nanoparticles. The particles may be sphericalnanoparticles and/or non-spherical, planar flakes. The alloy may beformed by at least one method selected from the group consisting of:atomization, pyrometallurgy, mechanical alloying, or combinationsthereof.

In a still further embodiment of the present invention, a composition isprovided having a precursor material comprising of: a) solid particlesof a first material comprising an alloy of a group IIIA-based materialand at least one group IA-based material and b) particles containing atleast one element from the group consisting of: group IB, IIIA, VIAelement, alloys containing any of the foregoing elements, orcombinations thereof. The group IA-based material is included in anamount sufficient so that no liquid phase of the alloy is present withinthe first material in a temperature range between room temperature and adeposition or pre-deposition temperature higher than room temperature,wherein the group IIIA-based material is otherwise liquid in thattemperature range. It should be understood that the group IA-basedmaterial comprises of Na.

For any of the embodiments described herein, the following may alsoapply. Optionally, the method may include formulating an ink includingthe precursor material; solution depositing the ink onto a substrate toform a precursor layer on the substrate; and reacting the precursorlayer in a suitable atmosphere to form a group IB-IIIA based film. Thismay be a two step process where the group IB-IIIA film may not include agroup VIA material and is further treated in a second step to form agroup IB-IIIA-VIA compound. The first film may be a dense film thatincludes a group IB-IIIA compound. The method may comprise of heatingthe film in a group VIA based atmosphere to form a group IB-IIIA-VIAcompound film. The film may comprise of a semiconductor film suitablefor use as an absorber layer in a photovoltaic device. The film may becomprised of an absorber layer for a solar cell. The reacting step maybe comprised of heating the precursor layer. In other embodiments, thereacting step comprises of heating the precursor layer in a groupVIA-based atmosphere. Optionally, the suitable atmosphere may contain atleast one of the following: selenium, sulfur, tellurium, H₂, CO, H₂Se,H₂S, Ar, N₂ or combinations or mixtures thereof. The method may includeadding a mixture of one or more elemental or alloy particles containingat least one element selected from the group consisting of: aluminum,tellurium, or sulfur. One or more classes of the particles may be dopedwith one or more inorganic materials. One or more classes of theparticles may be doped with one or more inorganic materials chosen fromthe group consisting of: aluminum (Al) and sulfur (S). One or moreclasses of the particles may be doped with one or more inorganicmaterials chosen from the group consisting of: sodium (Na), potassium(K), or lithium (Li). The alloy containing particles may be a solesource of group MA elements in the ink. In terms of composition, thefilm may have a Cu/(In+Ga) compositional range of about 0.01 to about1.0 and a Ga/(In+Ga) compositional range of about 0.01 to about 1.0. Thefilm may have a Cu/(In+Ga) compositional range of about >1.0 forCu/(In+Ga) and a Ga/(In+Ga) compositional range of about 0.01 to about1.0. The film may have a Cu/(In+Ga) compositional range of about 0.01 toabout 1.0 and a Ga/(In+Ga) compositional range of about 0.01 to about1.0. Optionally, the film has a desired Cu/(In+Ga) molar ratio is in therange of about 0.7 to about 1.0 and a desired Ga/(Ga+In) molar ratio inthe range of about 0.1 to about 0.8. Optionally, there is thepossibility of having a ratio of Cu/(In+Ga)>1.0 and using subsequentpost-treatment (KCN, etc.) to change Cu/(In+Ga)<1.0.

For any of the embodiments described herein, the following may alsoapply. In some embodiments, the ink includes a carrier liquid. Thedepositing step may include using at least one of the followingtechniques: wet coating, spray coating, spin coating, doctor bladecoating, contact printing, top feed reverse printing, bottom feedreverse printing, nozzle feed reverse printing, gravure printing,microgravure printing, reverse microgravure printing, comma directprinting, roller coating, slot die coating, meyerbar coating, lip directcoating, dual lip direct coating, capillary coating, ink jetprinting,jet deposition, spray deposition, or combinations thereof.

For any of the embodiments described herein, the following may alsoapply. The material may increase the melting temperature and does notcontain contaminants that require further heating to remove contaminantsadded by the material. The material may be included in an amountsufficient so that no liquid phase of the alloy is present within thefirst material in a temperature range between room temperature and adeposition or pre-deposition temperature higher than room temperature,wherein the material is otherwise liquid in that temperature range anddoes not require further heating to remove any materials added by theadditive.

In yet another embodiment of the present invention, the method includesproviding a first material comprising an alloy of a) a group IIIA-basedmaterial and b) a second material, wherein the second material isincluded in an amount sufficient so that no liquid phase of the alloy ispresent within the first material in a temperature range between roomtemperature and a deposition or pre-deposition temperature higher thanroom temperature, wherein the group IIIA-based material is otherwiseliquid in that temperature range. A precursor material may be formulatedcomprising of a) particles of the first material and b) particlescontaining at least one element from the group consisting of: group IB,IIIA, VIA element, alloys containing any of the foregoing elements, orcombinations thereof. The second material comprises of F. Optionally,the second material comprises of NO₃. The second material comprises ofany melting-point increasing material, relative to a melting point ofthe group IIIA-based material. The alloy may be comprised of GaF₃. Thealloy may also be comprised of Ga(NO₃)₃. The alloy may be comprised of agroup IIIA-based salt. The alloy may be comprised of an organo-galliumcompound. The method may include heating the precursor material to forma layer without C, N, O, or F elements in the layer. The second materialmay contain aluminum (Al) and/or aluminum compounds. The second materialmay contain sulfur (S) and/or sulfur compounds.

Alloys

In one embodiment of the present invention, a method is provided forcreating solid alloy particles. The method may include providing a firstmaterial containing at least one alloy comprising of: a) a group IIIAelement, b) at least one group IB, IIIA, and/or VIA element differentfrom the group IIIA element of a), and c) a group IA-based material. Thegroup IA-based material may be included in an amount sufficient so thatno liquid phase of the alloy is present in a temperature range betweenroom temperature and a deposition or pre-deposition temperature higherthan room temperature, wherein the group IIIA element is otherwiseliquid in that temperature range. The method may involve formulating aprecursor material comprising of: a) particles of the first material andb) particles containing at least one element from one of the following:a group IB element, a group IIIA element, a group VIA element, alloyscontaining any of the foregoing elements, or combinations thereof.

For any of the embodiments described herein, the following may alsoapply. The temperature range where the alloy is solid may be betweenabout 20° C. and about 200° C. The alloy may have a higher meltingtemperature than a melting temperature of the group IIIA element of a).The precursor material may further include a second material containinga group IB, IIIA, and/or VIA based material. There may further includeparticles containing the precursor material. The group IIIA element ofthe first material may be indium. The group IIIA element of the firstmaterial may be Ga. The group IA-based material may be at leastpartially included in the particles. The group IA-based material may becomprised of elemental sodium. The group IA-based material may becomprised of a sodium-based compound. The group IA-based material maycontain an element chosen from the group consisting of sodium (Na),potassium (K), lithium (Li), compounds containing any of the foregoing,or combinations thereof. The alloy may be comprised of In—Ga—Na,In—Ga—Se—Na, and/or Ga—Se—Na. Optionally, the alloy may be comprised ofone of the following: In—Se—Na, Cu—In—Na, or Cu—Ga—Na. The alloy may becomprised of a sulfide. The alloy may be comprised of Cu—In—Ga—Na. Theprecursor material contains particles comprising of Cu-based particles.The precursor material may contain particles comprising of Cu-basedalloy particles. The precursor material may contain particles comprisingof Cu-IIIA based alloy particles. The precursor material may containparticles comprising of Cu-VIA based alloy particles. The particles mayinclude nanoparticles. Optionally, the particles may include sphericalnanoparticles. The particles include non-spherical, planar flakes.

In one embodiment, the material may solidify substantially all of theparticle. Optionally, the alloy may solidify at least an outer portionof the particles to prevent leaching or phase separation of liquid groupIIIA element from the particles. The alloy may create a solid outershell on the particles to prevent leaching of liquid group IIIA elementfrom the particles. The particles may be formed by using at least one ofthe following methods: grinding, milling, electroexplosive wire (EEW)processing, evaporation condensation (EC), pulsed plasma processing, orcombinations thereof. The particles may be formed using at least one ofthe following methods: sonification, agitation, electromagneticallymixing of a liquid metal or liquid alloy. The particles may be formedusing at least one of the following methods: spray-pyrolysis, laserpyrolysis, or a bottom-up technique like wet chemical approaches.

For any of the embodiments described herein, the following may alsoapply. The particles may be nanoparticles. The particles may bespherical nanoparticles. Optionally, at least some of the particles arenon-spherical, planar flakes. The method may include using the precursormaterial in a solution coatable ink for forming a film on a substrate.The method may include formulating an ink including the precursormaterial; solution depositing the ink onto a substrate to form aprecursor layer on the substrate; and reacting the precursor layer in asuitable atmosphere to form a group IB-IIIA-VIA based film. In otherembodiments, the method may include formulating an ink including theprecursor material; solution depositing the ink onto a substrate to forma precursor layer on the substrate; and reacting the precursor layer ina suitable atmosphere to form a IB-IIIA film. The film may include agroup IB-IIIA-VIA compound. The film may be a dense film that includes agroup IB-IIIA compound. The film may be heated in a group VIA basedatmosphere to form a group IB-IIIA-VIA compound film. The film may becomprised of a semiconductor film suitable for use in a photovoltaicdevice. The film may be comprised of an absorber layer for a solar cell.The reacting step may be comprised of heating the layer in the suitableatmosphere. The method may include adding a mixture of one or moreelemental particles selected from: aluminum, tellurium, sulfur, copper,indium, gallium, alloys containing any of the foregoing, andcombinations thereof. The suitable atmosphere may contain at least oneof the following: selenium, sulfur, tellurium, H2, CO, H2Se, H2S, Ar, N2or combinations or mixtures thereof. Optionally, one or more classes ofthe particles may be doped with one or more inorganic materials. One ormore classes of the particles may be doped with one or more inorganicmaterials containing at least one element from the group of aluminum(Al), sulfur (S), sodium (Na), potassium (K), or lithium (Li).

For any of the embodiments described herein, the following may alsoapply. Optionally, the alloy containing particles may be a sole sourceof group IIIA elements in the ink. The film may have a Cu/(In+Ga)compositional range of about 0.01 to about 1.0 and a Ga/(In+Ga)compositional range of about 0.01 to about 1.0. The film may have aCu/(In+Ga) compositional range>1.0 and a Ga/(In+Ga) compositional rangeof about 0.01 to about 1.0. The film may have a desired Cu/(In+Ga) molarratio is in the range of about 0.7 to about 1.0 and a desired Ga/(Ga+In)molar ratio in the range of about 0.1 to about 0.8. The ink may includea carrier liquid. Depositing the material may be comprised of using atleast one of the following techniques: wet coating, spray coating, spincoating, doctor blade coating, contact printing, top feed reverseprinting, bottom feed reverse printing, nozzle feed reverse printing,gravure printing, microgravure printing, reverse microgravure printing,comma direct printing, roller coating, slot die coating, meyerbarcoating, lip direct coating, dual lip direct coating, capillary coating,ink jet printing, jet deposition, spray deposition, or combinationsthereof.

In yet another embodiment of the present invention, a composition isprovided that comprises of a precursor material comprising of: a) solidparticles of a first material and b) particles containing at least oneelement from the group consisting of: group IB, IIIA, VIA element,alloys containing any of the foregoing elements, or combinationsthereof. The first material may contain at least one alloy comprised of:a) a group IIIA element, b) at least one group IB, IIIA, and/or VIAelement different from the group IIIA element of a), and c) a groupIA-based material. The group IA-based material is included in an amountsufficient so that no liquid phase of the alloy is present in atemperature range between room temperature and a deposition orpre-deposition temperature higher than room temperature, wherein thegroup IIIA element is otherwise liquid in that temperature range. Thegroup IA-based material may be comprised of Na. The composition mayinclude any of features previously discussed in the foregoingparagraphs.

In a still further embodiment of the present invention, a methodincludes providing a first material containing at least one alloycomprising: a) a group IIIA element, b) at least one group IB, IIIA,and/or VIA element different from the group IIIA element of a), and c) asecond material. The second material is included in an amount sufficientso that no liquid phase of the alloy is present in a temperature rangebetween room temperature and a deposition or pre-deposition temperaturehigher than room temperature, wherein the group IIIA element isotherwise liquid in that temperature range. The method may includeformulating a precursor material comprising a) particles of the firstmaterial and b) particles containing at least one element from the groupconsisting of: group IB, IIIA, VIA element, alloys containing any of theforegoing elements, or combinations thereof.

For any of the embodiments described herein, the following may alsoapply. The second material may be comprised of F. Optionally, the secondmaterial may be comprised of NO₃. The second material may include anymelting-point increasing material for increasing the melting pointrelative to a melting point of the group IIIA-based material. The alloymay be comprised of GaF₃ and/or Ga(NO₃)₃. Optionally, the alloy may becomprised of a group IIIA-based salt. The alloy may be comprised of anorgano-gallium compound. The precursor material may be heated to form alayer without C, N, O, or F elements in the layer. The second materialmay contain aluminum (Al) and/or aluminum compounds. The second materialmay contain sulfur (S) and/or sulfur compounds.

Quenching

In one embodiment of the present invention, a process for forming solidparticles is provided. The method includes providing a first suspensionof solid and/or liquid particles containing at least one group IIIAelement. A material may be added to substantially increase the meltingpoint of at least one set of group IIIA-containing particles in thesuspension into higher-melting solid particles comprising an alloy ofthe group IIIA element and at least a part of the added material. Thesuspension may be deposited onto a substrate to form a precursor layeron the substrate and the precursor layer is reacted in a suitableatmosphere to form a film.

For any of the embodiments described herein, the following may alsoapply. The alloy may have a higher melting temperature than a meltingtemperature of the IIIA element. The solid and/or liquid particlescontain at least one element from the group consisting of: group IB,IIIA, VIA element, alloys containing any of the foregoing elements, orcombinations. A second suspension may be provided, wherein the secondsuspension includes solid and/or liquid particles containing at leastone element from the group consisting of: group IB, IIIA, VIA element,alloys containing any of the foregoing elements, or combinations. Thematerial may be added to create solid particles of the material and thegroup IIIA element. The first suspension may be separately preparedbefore mixing it with a second suspension. AIIIA-alloy-solid-particles-based suspension may be separately preparedbefore mixing it with the other IB and/or IIIA and/or VIA elements. Themethod may involve separate emulsion/suspension creation step beforeadding it to the mixed final suspension and depositing the suspensiononto a substrate to form a precursor layer on the substrate. Optionally,at least one set of the solid particles are group IIIA-Na alloycontaining particles, wherein Na in the group IIIA-Na alloy containingparticles is at an amount sufficient so that no liquid phase of a groupIIIA-Na alloy is present within the group IIIA-Na alloy containingparticles in a temperature range between room temperature and adeposition or pre-deposition temperature higher than room temperature,wherein the group IIIA-based material is otherwise liquid in thattemperature range. Optionally, at least one set of the solid particlesare group IIIA-Na alloy containing particles, wherein Na in the groupIIIA-Na alloy containing particles is at an amount sufficient so that noliquid phase of a group IIIA-Na alloy is present within the groupIIIA-Na alloy containing particles in a temperature range between about15 C and about 200 C, wherein the group IIIA-based material is otherwiseliquid in that temperature range. Optionally, at least one set of thesolid particles are group IIIA-Na alloy containing particles, wherein Nain the group IIIA-Na alloy containing particles is at an amountsufficient so that no liquid phase of a group IIIA-Na alloy is presentwithin the group IIIA-Na alloy containing particles at a depositionand/or dispersion temperature. The suspension may be cooled to solidifythe particles. The depositing step may be comprised of solutiondepositing the suspension.

For any of the embodiments described herein, the following may alsoapply. The material may be comprised of elemental sodium and/or asodium-based material. The material may be comprised of theaforementioned in liquid and/or solid form. The adding step may becomprised of adding an emulsion of the material to an emulsioncontaining a liquid group IIIA element to create the solid particles.The adding step may be comprised of adding an emulsion of the materialto dispersion of solid group IIIA element to create the solid particles.Optionally, the adding step comprises of adding a dispersion of solidmaterial particles of the material to an emulsion containing a liquidgroup IIIA element to create the solid particles. The adding step may becomprised of adding a dispersion of solid material particles of thematerial to a dispersion of solid particle containing a group IIIAelement for a solid-solid reaction to create solid particles. The methodmay include milling the material and the one set of groupIIIA-containing liquid particles in the suspension to more thoroughlymix solids with the liquid. Optionally, the method may includemechanically alloying the material and the one set of groupIIIA-containing particles in the suspension to more thoroughly mixsolids. Adding the material may create particles of sizes smaller thanthe size of the group IIIA-containing particles found in the suspensionprior to introduction of the material. The method may include agitatingthe suspension to mix and size reduce the particles. The suspension maybe sonicated to mix and size reduce the particles. Electromagneticsize-reduction/control may be used to mix and size reduce the particles.Any sodium containing particles may be added during emulsification tosolidify droplets of Ga to form solid Ga—Na particles. Sodium inelemental form may be added prior to, during, or after emulsification tosolidify droplets of Ga to form solid Ga—Na particles. Liquid sodium maybe added in elemental form prior to, during, or after emulsification tosolidify droplets of Ga to form solid Ga—Na particles. A sodium emulsionmay be combined with a gallium emulsion to solidify droplets of Ga toform solid Ga—Na particles. A sodium emulsion may be combined with agallium emulsion by milling to solidify droplets of Ga to form solidGa—Na particles. A sodium emulsion may be combined with a solid galliumparticles by mechanical alloying at temperatures below the melting pointof gallium to form solid Ga—Na particles. A sodium dispersion may becombined with a gallium dispersion by mechanical alloying to solidifydroplets of Ga to form solid Ga—Na particles. The film may include agroup IB-IIIA-VIA compound. The reacting step may be comprised ofheating the layer in a suitable atmosphere.

For any of the embodiments described herein, the following may alsoapply. Optionally, at least one set of the particles in the suspensionis in the form of nanoglobules. In other embodiments, at least one setof the particles in the suspension are in the form of nanoglobules andcontain at least one group IIIA element. At least one set of theparticles in the suspension may be in the form of nanoglobulescomprising of a group IIIA element in elemental form. At least some ofthe particles may have a platelet shape. Optionally, a majority of theparticles may have a platelet shape. All of the particles may have aplatelet shape. The particles may have a substantially flat, planarshape. A majority of the particles may have a flat, planar shape. All ofthe particles may have a flat, planar shape. The depositing step mayinclude coating the substrate with the suspension. The suspension may becomprised of an emulsion. Gallium may be incorporated as a group IIIAelement in the form of a suspension of nanoglobules. Nanoglobules ofgallium may be formed by creating an emulsion of liquid gallium in asolution. A suspension of liquid gallium in solution may be maintainedor enhanced by stirring, mechanical means, electromagnetic means,ultrasonic means, and/or the addition of dispersants and/or emulsifiers.

For any of the embodiments described herein, the following may alsoapply. A mixture of one or more elemental particles may be added,wherein the particles are selected from: aluminum, tellurium, and/orsulfur. The suitable atmosphere may contain at least one of thefollowing: selenium, sulfur, tellurium, H₂, CO, H₂Se, H₂S, Ar, N₂ orcombinations or mixture thereof. One or more classes of the particlesmay include one or more inorganic materials. The particles may becontain one or more inorganic materials chosen from the group consistingof: aluminum (Al), sulfur (S), sodium (Na), potassium (K), lithium (Li),alloys containing the foregoing elements, or combinations thereof. Theparticles may be nanoparticles. Particles may be formed from a feedstockby one of the following processes: milling, electroexplosive wire (EEW)processing, evaporation condensation (EC), pulsed plasma processing, orcombinations thereof. Optionally, the material does negatively impactthe resulting absorber layer and not need to be removed from theresulting absorber layer. The material may be comprised of Al to makesolid Al—Ga particles. Optionally, the process may be comprised of amaterial comprising of Al, wherein Ga dissolves in Al to make solidAl—Ga particles for use in forming a film of CAGS and/or CAIGS.

Bandgap

In one embodiment, a method is provided for bandgap grading in athin-film device using such particles. The method may be comprised ofproviding a bandgap grading material comprising of an alloy having: a) aIIIA material and b) a group IA-based material, wherein the alloy has ahigher melting temperature than a melting temperature of the IIIAmaterial in elemental form. A precursor material may be deposited on asubstrate to form a precursor layer. The precursor material comprisinggroup IB, IIIA, and/or VIA based particles. The bandgap grading materialof the alloy may be deposited after depositing the precursor material.The alloy in the grading material may react after the precursor layerhas begun to sinter and thus maintains a higher concentration of IIIAmaterial in a portion of the compound film that forms above a portionthat sinters first.

For any of the embodiments described herein, the following may alsoapply. The bandgap grading material may melt above 450° C. Optionally,the bandgap grading material melts above 500° C. In another embodiment,the bandgap grading material melts above 550° C. The method may includeat least partially sintering the precursor material to form a dense filmprior to depositing the bandgap grading material. The precursor materialmay be completely sintered to form a dense film prior to depositing thebandgap grading material. The precursor material may be reacted in asuitable atmosphere to form a CIS-based film prior to depositing thebandgap grading material. The precursor material and the bandgap gradingmaterial may be reacted in a suitable atmosphere to form a CIGS film.The alloy may have a higher reacting temperature than a maximumsintering temperature of the precursor material. The depositing step maybe comprised of solution depositing the precursor material. Optionally,the depositing step may be comprised of dry powder depositing theprecursor material. The reacting step comprises of using a solid-statereaction. The reacting step comprises of heating the precursor materialat a first temperature profile, wherein the precursor layer at leastpartially sinters at the first temperature profile where a maximumtemperature is lower than a reacting temperature of the alloy; andincreasing processing temperature to a second temperature sufficient tomelt react the particles of the alloy, wherein the alloy reacts afterthe precursor layer has begun to at least partially sinter. The bandgapgrading material may be solution deposited over the precursor layer. Thebandgap grading material may be deposited over the precursor layer usinga vacuum-based technique. The bandgap grading material may be depositedover the precursor layer by sputtering. The bandgap grading material maybe deposited over the precursor layer by at least one of the followingtechniques: ALD, CVD, PVD, or combinations thereof electrodeposition,solution-deposition of ‘moleculary’ soluble Ga-compounds in contrast toparticles=aggregates). The precursor material may be comprised of amaterial that forms a Cu—In—Se based alloy when sintered at the firsttemperature profile. The precursor material may be comprised of aprecursor material that forms a Cu—In—Se based alloy when sintered atthe first temperature profile and combines with the alloy to form aCu—In—Ga—Se based alloy when processed with the second temperatureprofile.

For any of the embodiments described herein, the following may alsoapply. The precursor material may be comprised of a material that formsa Cu—In—Ga—Se based alloy when sintered at the first temperature profileand combines with the alloy to form a Cu—In—Ga—Se based alloy whenprocessed with the second temperature profile with increase Ga contentnear a top surface of the layer. The alloy may be comprised of a Ga—Nabased material, a Ga—Na—Se based material, a Ga—Na—S based material,and/or a Ga—Na—Te based material. The group IA-based material may becomprised of elemental sodium-based material. The group IA-basedmaterial may be comprised of a sodium-based compound. The group IA-basedmaterial may be chosen from the group of sodium (Na), potassium (K),lithium (Li), alloys containing any of the foregoing, or combinationsthereof. The precursor material may contain particles comprised ofCu-based alloy particles. The precursor material contains particlescomprised of Cu-IIIA based alloy particles. The precursor material maycontain particles comprised of Cu-VIA based alloy particles. Theprecursor material may be comprised of a selenide-based alloy. Theparticles may be nanoparticles. The particles may be sphericalnanoparticles. The particles may include non-spherical, planar flakes.The compound film may include a group IB-IIIA-VIA compound. The compoundfilm may be comprised of a semiconductor film suitable for use in aphotovoltaic device. The compound film may be comprised of an absorberlayer for a solar cell. A mixture of one or more elemental particles maybe added and selected from: aluminum, tellurium, or sulfur. The suitableatmosphere may contain at least one of the following: selenium, sulfur,tellurium, H2, CO, H2Se, H2S, Ar, N2 or combinations or mixturesthereof. One or more classes of the particles may be doped with one ormore inorganic materials. One or more classes of the particles may bedoped with one or more inorganic materials chosen from the group ofaluminum (Al), sulfur (S), sodium (Na), potassium (K), or lithium (Li).The film may have a Cu/(In+Ga) compositional range of about 0.01 toabout 1.0 and a Ga/(In+Ga) compositional range of about 0.01 to about1.0. The film may have a desired Cu/(In+Ga) molar ratio in the range ofabout 0.7 to about 1.0 and a desired Ga/(Ga+In) molar ratio in the rangeof about 0.1 to about 0.8. The film may optionally have a desiredCu/(In+Ga) molar ratio in the range of greater than about 1.0 and adesired Ga/(Ga+In) molar ratio in the range of about 0.1 to about 0.8.The film may be reacted in a post-reacting step to change Cu/(In+Ga) tobe in a range less than about 1.0. Solution deposition comprises usingat least one of the following techniques: wet coating, spray coating,spin coating, doctor blade coating, contact printing, top feed reverseprinting, bottom feed reverse printing, nozzle feed reverse printing,gravure printing, microgravure printing, reverse microgravure printing,comma direct printing, roller coating, slot die coating, meyerbarcoating, lip direct coating, dual lip direct coating, capillary coating,ink jetprinting, jet deposition, spray deposition, or combinationsthereof.

In yet another embodiment of the present invention, a method is providedfor bandgap grading. The method may be comprised of providing a bandgapgrading material having an alloy of a) an group IA-based material and b)Ga. The particles of the alloy may be deposited over a previously formedCu—In—Ga—Se based layer. The particles with the previously formedCu—In—Ga—Se based layer may be reacted in a suitable atmosphere at aprocessing temperature, wherein the bandgap grading material is reactedto form a gallium-rich portion of the Cu—In—Ga—Se based layer over atleast a portion of the previously formed Cu—In—Ga—Se based layer.

For any of the embodiments described herein, the following may alsoapply. The group IA-based material may be comprised of an Na-basedmaterial. The group IA-based material may be comprised of elemental Na.The alloy may be comprised of a Ga—Na based material. The alloy may becomprised of a Ga—Na—Se based material. It should be understood that anyof the materials used herein are not limited to solution deposition butmay also be suitable for deposition using vacuum-based techniques.

Inter-Metallics

In one embodiment, the method comprises forming a precursor layer on asubstrate, wherein the precursor layer comprises one or more discretelayers. The layers may include at least a first layer containing one ormore group IB elements and two or more different group IIIA elements andat least a second layer containing elemental chalcogen particles. Theprecursor layer may be heated to a temperature sufficient to melt thechalcogen particles and to react the chalcogen particles with the one ormore group IB elements and group IIIA elements in the precursor layer toform a film of a group IB-IIIA-chalcogenide compound. The method mayalso include making a film of group IB-IIIA-chalcogenide compound thatincludes mixing the nanoparticles and/or nanoglobules and/ornanodroplets to form an ink, depositing the ink on a substrate, heatingto melt the extra chalcogen and to react the chalcogen with the group IBand group IIIA elements and/or chalcogenides to form a dense film. Insome embodiments, densification of the precursor layer is not used sincethe absorber layer may be formed without first sintering the precursorlayer to a temperature where densification occurs. At least one set ofthe particles in the precursor layer are inter-metallic particlescontaining at least one group IB-IIIA inter-metallic alloy phase.Alternatively, at least one set of the particles in the precursor layerare formed from a feedstock of inter-metallic particles containing atleast one group IB-IIIA inter-metallic alloy phase.

For any of the embodiments described herein, the following may alsoapply. Optionally, the first layer may be formed over the second layer.In another embodiment, the second layer may be formed over the firstlayer. The first layer may also contain elemental chalcogen particles.The first layer may have group IB elements in the form of a groupIB-chalcogenide. The first layer may have group IIIA elements in theform of a group IIIA-chalcogenide. There may be a third layer containingelemental chalcogen particles. The two or more different group IIIAelements may include indium and gallium. The group IB element may becopper. The chalcogen particles may be particles of selenium, sulfur,and/or tellurium. The precursor layer may be substantially oxygen-free.Forming the precursor layer may include forming a dispersion includingnanoparticles containing one or more group IB elements and nanoparticlescontaining two or more group IIIA elements, spreading a film of thedispersion onto the substrate. Forming the precursor layer may includesintering the film to form the precursor layer. Sintering the precursorlayer may take place before the step of disposing the layer containingelemental chalcogen particles over the precursor layer. The substratemay be a flexible substrate and wherein forming the precursor layerand/or disposing the layer containing elemental chalcogen particles overthe precursor layer, and/or heating the precursor layer and chalcogenparticles includes the use of roll-to-roll manufacturing on the flexiblesubstrate. The substrate may be an aluminum foil substrate. The groupIB-IIIA-chalcogenide compound may be of the formCuzIn(1-x)GaxS2(1-y)Se2y, where 0.5≦z≦1.5, 0≦x≦1.0 and 0≦y≦1.0. Inanother embodiment of the present invention, heating of precursor layerand chalcogen particles may include heating the substrate and precursorlayer from an ambient temperature to a plateau temperature range ofbetween about 200° C. and about 600° C., maintaining a temperature ofthe substrate and precursor layer in the plateau range for a period oftime ranging between about a fraction of a second to about 60 minutes,and subsequently reducing the temperature of the substrate and precursorlayer.

In a still further embodiment of the present invention, a method isprovided for forming a film of a group IB-IIIA-chalcogenide compound.The method includes forming a precursor layer on a substrate, whereinthe precursor layer contains one or more group IB elements and one ormore group IIIA elements. The method may include sintering the precursorlayer. After sintering the precursor layer, the method may includeforming a layer containing elemental chalcogen particles over theprecursor layer. The method may also include heating the precursor layerand chalcogen particles to a temperature sufficient to melt thechalcogen particles and to react the chalcogen particles with the groupIB element and group IIIA elements in the precursor layer to form a filmof a group IB-IIIA-chalcogenide compound. The one or more group IIIAelements may include indium and gallium. The chalcogen particles may beparticles of selenium, sulfur or tellurium. The precursor layer may besubstantially oxygen-free. The method may include forming the precursorlayer which includes forming a dispersion containing nanoparticlescontaining one or more group IB elements and nanoparticles containingtwo or more group IIIA elements, spreading a film of the dispersion ontoa substrate. The method may include forming the precursor layer and/orsintering the precursor layer and/or disposing the layer containingelemental chalcogen particles over the precursor layer and/or heatingthe precursor layer and chalcogen particles to a temperature sufficientto melt the chalcogen particles includes the use of roll-to-rollmanufacturing on the flexible substrate. The group IB-IIIA-chalcogenidecompound may be of the form CuzIn(1−x)GaxS2(1−y)Se2y, where 0.5≦z≦1.5,0≦x≦1.0 and 0≦y≦1.0.

For any of the embodiments described herein, the following may alsoapply. Sintering the precursor layer may include heating the substrateand precursor layer from an ambient temperature to a plateau temperaturerange of between about 200° C. and about 600° C., maintaining atemperature of the substrate and precursor layer in the plateau rangefor a period of time ranging between about a fraction of a second toabout 60 minutes, and subsequently reducing the temperature of thesubstrate and precursor layer. Heating the precursor layer and chalcogenparticles may include heating the substrate, precursor layer, andchalcogen particles from an ambient temperature to a plateau temperaturerange of between about 200° C. and about 600° C., maintaining atemperature of the substrate and precursor layer in the plateau rangefor a period of time ranging between about a fraction of a second toabout 60 minutes, and subsequently reducing the temperature of thesubstrate and precursor layer. It should also be understood that thesubstrate may be an aluminum foil substrate.

In a still further embodiment of the present invention, a method isprovided that is comprised of forming a precursor layer containingparticles having one or more group IB elements and two or more differentgroup IIIA elements and forming a layer containing surplus chalcogenparticles providing a source of excess chalcogen, wherein the precursorlayer and the surplus chalcogen layer are adjacent to one another. Theprecursor layer and the surplus chalcogen layer are heated to atemperature sufficient to melt the particles providing the source ofexcess chalcogen and to react the particles with the one or more groupIB elements and group IIIA elements in the precursor layer to form afilm of a group IB-IIIA-chalcogenide compound on a substrate. Thesurplus chalcogen layer may be formed over the precursor layer. Thesurplus chalcogen layer may be formed under the precursor layer. Theparticles providing the source of excess chalcogen may be comprised ofelemental chalcogen particles. The particles providing the source ofexcess chalcogen may be comprised of chalcogenide particles. Theparticles providing the source of excess chalcogen may be comprised ofchalcogen-rich chalcogenide particles. The precursor layer may alsocontain elemental chalcogen particles. The precursor layer may havegroup IB elements in the form of a group IB-chalcogenide. The precursorlayer may have group IIIA elements in the form of a groupIIIA-chalcogenide. A third layer may be provided that contains elementalchalcogen particles. The film may be formed from the precursor layer ofthe particles and a layer of a sodium-containing material in contactwith the precursor layer.

For any of the embodiments described herein, the following may alsoapply. Optionally, the film may be formed from a precursor layer of theparticles and a layer in contact with the precursor layer and containingat least one of the following materials: a group IB element, a groupIIIA element, a group VIA element, a group IA element, a binary and/ormultinary alloy of any of the preceding elements, a solid solution ofany of the preceding elements, copper, indium, gallium, selenium, copperindium, copper gallium, indium gallium, sodium, a sodium compound,sodium fluoride, sodium indium sulfide, copper selenide, copper sulfide,indium selenide, indium sulfide, gallium selenide, gallium sulfide,copper indium selenide, copper indium sulfide, copper gallium selenide,copper gallium sulfide, indium gallium selenide, indium gallium sulfide,copper indium gallium selenide, and/or copper indium gallium sulfide. Inone embodiment, the particles contain sodium at about 1 at. % or less.The particles may contain at least one of the following materials:Cu—Na, In—Na, Ga—Na, Cu—In—Na, Cu—Ga—Na, In—Ga—Na, Na—Se, Cu—Se—Na,In—Se—Na, Ga—Se—Na, Cu—In—Se—Na, Cu—Ga—Se—Na, In—Ga—Se—Na,Cu—In—Ga—Se—Na, Na—S, Cu—S—Na, In—S—Na, Ga—S—Na, Cu—In—S—Na, Cu—Ga—S—Na,In—Ga—S—Na, or Cu—In—Ga—S—Na. The film may be formed from a precursorlayer of the particles and an ink containing a sodium compound with anorganic counter-ion or a sodium compound with an inorganic counter-ion.Optionally, the film may be formed from a precursor layer of theparticles and a layer of a sodium containing material in contact withthe precursor layer and/or particles containing at least one of thefollowing materials: Cu—Na, In—Na, Ga—Na, Cu—In—Na, Cu—Ga—Na, In—Ga—Na,Na—Se, Cu—Se—Na, In—Se—Na, Ga—Se—Na, Cu—In—Se—Na, Cu—Ga—Se—Na,In—Ga—Se—Na, Cu—In—Ga—Se—Na, Na—S, Cu—S—Na, In—S—Na, Ga—S—Na,Cu—In—S—Na, Cu—Ga—S—Na, In—Ga—S—Na, or Cu—In—Ga—S—Na; and/or an inkcontaining the particles and a sodium compound with an organiccounter-ion or a sodium compound with an inorganic counter-ion. Themethod may also include adding a sodium containing material to the filmafter the heating step.

For any of the embodiments described herein, the following may alsoapply. A liquid ink may be made using one or more liquid metals. Forexample, an ink may be made starting with a liquid and/or molten mixtureof Gallium and/or Indium. Copper nanoparticles may then be added to themixture, which may then be used as the ink/paste. Copper nanoparticlesare available commercially. Alternatively, the temperature of theCu—Ga—In mixture may be adjusted (e.g. cooled) until a solid forms. Thesolid may be ground at that temperature until small nanoparticles (e.g.,less than 5 nm) are present. Selenium may be added to the ink and/or afilm formed from the ink by exposure to selenium vapor, e.g., before,during, or after annealing.

In another embodiment, a liquid ink may be made using one or more liquidmetals. For example, an ink may be made starting with a liquid and/ormolten mixture of Gallium and/or Indium. Copper nanoparticles may thenbe added to the mixture, which may then be used as the ink/paste. Coppernanoparticles are available commercially. Alternatively, the temperatureof the Cu—Ga—n mixture may be adjusted (e.g. cooled) until a solidforms. The solid may be ground at that temperature until smallnanoparticles (e.g., less than 5 nm) are present. Selenium may be addedto the ink and/or a film formed from the ink by exposure to seleniumvapor, e.g., before, during, or after annealing.

In yet another embodiment of the present invention, a process isdescribed comprising of formulating a dispersion of solid and/or liquidparticles comprising group IB and/or IIIA elements, and, optionally, atleast one group VIA element. The process includes depositing thedispersion onto a substrate to form a layer on the substrate andreacting the layer in a suitable atmosphere to form a film. In thisprocess, at least one set of the particles are inter-metallic particlescontaining at least one group IB-IIIA inter-metallic phase.

In yet another embodiment of the present invention, a composition isprovided comprised of a plurality of particles comprising group IBand/or IIIA elements, and, optionally, at least one group VIA element.At least one set of the particles contains at least one group IB-IIIAinter-metallic alloy phase.

For any of the embodiments described herein, the following may alsoapply. The method may include formulating a dispersion of particlescomprising group IB and/or IIIA elements, and, optionally, at least onegroup VIA element. The method may include depositing the dispersion ontoa substrate to form a layer on the substrate and reacting the layer in asuitable atmosphere to form a film. At least one set of the particlescontain a group IB-poor, group IB-IIIA alloy phase. In some embodiments,group IB-poor particles contribute less than about 50 molar percent ofgroup IB elements found in all of the particles. The group IB-poor,group IB-IIIA alloy phase particles may be a sole source of one of thegroup IIIA elements. The group IB-poor, group IB-IIIA alloy phaseparticles may contain an inter-metallic phase and may be a sole sourceof one of the group IIIA elements. The group IB-poor, group IB-IIIAalloy phase particles may contain an inter-metallic phase and are a solesource of one of the group IIIA elements. The group IB-poor, groupIB-IIIA alloy phase particles may be Cu₁In₂ particles and are a solesource of indium in the material.

For any of the embodiments described herein, the following may alsoapply. It should be understood that for any of the foregoing, the filmand/or final compound may include a group IB-IIIA-VIA compound. Thereacting step may comprise of heating the layer in the suitableatmosphere. The depositing step may include coating the substrate withthe dispersion. At least one set of the particles in the dispersion maybe in the form of nanoglobules. At least one set of the particles in thedispersion may be in the form of nanoglobules and contain at least onegroup IIIA element. At least one set of the particles in the dispersionmay be in the form of nanoglobules comprising of a group IIIA element inelemental form. In some embodiments of the present invention, theinter-metallic phase is not a terminal solid solution phase. In someembodiments of the present invention, the inter-metallic phase is not asolid solution phase. The inter-metallic particles may contribute lessthan about 50 molar percent of group IB elements found in all of theparticles. The inter-metallic particles may contribute less than about50 molar percent of group IIIA elements found in all of the particles.The inter-metallic particles may contribute less than about 50 molarpercent of the group IB elements and less than about 50 molar percent ofthe group IIIA elements in the dispersion deposited on the substrate.The inter-metallic particles may contribute less than about 50 molarpercent of the group IB elements and more than about 50 molar percent ofthe group IIIA elements in the dispersion deposited on the substrate.The inter-metallic particles may contribute more than about 50 molarpercent of the group IB elements and less than about 50 molar percent ofthe group IIIA elements in the dispersion deposited on the substrate.The molar percent for any of the foregoing may be based on a total molarmass of the elements in all particles present in the dispersion. In someembodiments, at least some of the particles have a platelet shape. Insome embodiments, a majority of the particles have a platelet shape. Inother embodiments, substantially all of the particles have a plateletshape.

For any of the embodiments described herein, the following may alsoapply. For any of the foregoing embodiments, an inter-metallic materialfor use with the present invention is a binary material. Theinter-metallic material may be a ternary material. It may be a copperrich (or group IB rich) ternary or binary. It may be a copper poor (orgroup IB poor) ternary or copper poor binary, wherein additional copper(or group IB material) may be added from a different source. The copperpoor (or group IB poor) ternary or binary may contribute less than about50% of the total group IB material in the precursor and/or final film.The copper poor (or group IB poor) ternary or binary may contribute lessthan about 40% of the total group IB material in the precursor and/orfinal film. The copper poor (or group IB poor) ternary or binary maycontribute less than about 30% of the total group IB material in theprecursor and/or final film. The copper poor (or group IB poor) ternaryor binary may contribute less than about 20% of the total group IBmaterial in the precursor and/or final film. The copper poor (or groupIB poor) ternary or binary may contribute less than about 10% of thetotal group IB material in the precursor and/or final film. Theinter-metallic material may comprise of Cu₁In₂. The inter-metallicmaterial may be comprised of a composition in a δ phase of Cu₁In₂. Theinter-metallic material may be comprised of a composition in between a δphase of Cu₁In₂ and a phase defined by Cu16In9. The inter-metallicmaterial may be comprised of Cu₁G₂. The inter-metallic material may becomprised of an intermediate solid-solution of Cu₁G₂. The inter-metallicmaterial may be comprised of Cu₆₈Ga₃₈. The inter-metallic material maybe comprised of Cu₇₀Ga₃₀. The inter-metallic material may be comprisedof Cu₇₅Ga₂₅. The inter-metallic material may be comprised of acomposition of Cu—Ga of a phase in between the terminal solid-solutionand an intermediate solid-solution next to it. The inter-metallic may becomprised of a composition of Cu—Ga in a γ1 phase (about 31.8 to about39.8 wt % Ga). The inter-metallic may be comprised of a composition ofCu—Ga in a γ2 phase (about 36.0 to about 39.9 wt % Ga). Theinter-metallic may be comprised of a composition of Cu—Ga in a γ3 phase(about 39.7 to about −44.9 wt % Ga). The inter-metallic may be comprisedof a composition of Cu—Ga in a phase between γ2 and γ3. Theinter-metallic may be comprised of a composition of Cu—Ga in a phasebetween the terminal solid solution and γ1. The inter-metallic may becomprised of a composition of Cu—Ga in a θ phase (about 66.7 to about68.7 wt % Ga). The inter-metallic material may be comprised of Cu-richCu—Ga. Gallium may be incorporated as a group IIIA element in the formof a suspension of nanoglobules. Nanoglobules of gallium may be formedby creating an emulsion of liquid gallium in a solution. Galliumnanoglobules may be created by being quenched below room temperature.

For any of the embodiments described herein, the following may alsoapply. A process according to the any of the foregoing embodiments ofthe present invention may include maintaining or enhancing a dispersionof liquid gallium in solution by stirring, mechanical means,electromagnetic means, ultrasonic means, and/or the addition ofdispersants and/or emulsifiers. The process may include adding a mixtureof one or more elemental particles selected from: aluminum, tellurium,or sulfur. The suitable atmosphere may contain selenium, sulfur,tellurium, H₂, CO, H₂Se, H₂S, Ar, N₂ or combinations or mixture thereof.The suitable atmosphere may contain at least one of the following: H₂,CO, Ar, and N₂. One or more classes of the particles may be doped withone or more inorganic materials. Optionally, one or more classes of theparticles are doped with one or more inorganic materials chosen from thegroup of aluminum (Al), sulfur (S), sodium (Na), potassium (K), orlithium (Li). Optionally, embodiments of the present invention mayinclude having a copper source that does not immediately alloy with In,and/or Ga. One option would be to use (slightly) oxidized copper. Theother option would be to use CuxSey. Note that for the (slightly)oxidized copper approach, a reducing step may be desired. Basically, ifelemental copper is used in liquid In and/or Ga, speed of the processbetween ink preparation and coating should be sufficient so that theparticles have not grown to a size that will result in thicknessnon-uniform coatings. It should be understood that the temperature rangemay that of the substrate only since that is typically the only one thatshould not be heated above its melting point. This holds for the lowestmelting material in the substrate, being Al and other suitablesubstrates.

For any of the embodiments described herein, the following may alsoapply. In one embodiment of the present invention, the method comprisesforming a precursor material comprising group IB and/or group IIIAparticles of any shape. The method may include forming a precursor layerof the precursor material over a surface of a substrate. The method mayfurther include heating the particle precursor material in asubstantially oxygen-free chalcogen atmosphere to a processingtemperature sufficient to react the particles and to release chalcogenfrom the chalcogenide particles, wherein the chalcogen assumes a liquidform and acts as a flux to improve intermixing of elements to form agroup IB-IIIA-chalcogenide film at a desired stoichiometric ratio. Thechalcogen atmosphere may provide a partial pressure greater than orequal to the vapor pressure of liquid chalcogen in the precursor layerat the processing temperature. This may be used in a one stage processor a two stage process.

For any of the embodiments described herein, the following may alsoapply. In one embodiment of the present invention, the method comprisesforming a precursor material comprising group IB and/or group IIIAand/or group VIA particles of any shape. The method may include forminga precursor layer of the precursor material over a surface of asubstrate. The method may further include heating the particle precursormaterial in a substantially oxygen-free chalcogen atmosphere to aprocessing temperature sufficient to react the particles and to releasechalcogen from the chalcogenide particles, wherein the chalcogen assumesa liquid form and acts as a flux to improve intermixing of elements toform a group IB-IIIA-chalcogenide film at a desired stoichiometricratio. The suitable atmosphere may be a selenium atmosphere. Thesuitable atmosphere may comprise of a selenium atmosphere providing apartial pressure greater than or equal to vapor pressure of selenium inthe precursor layer. The suitable atmosphere may comprise of anon-oxygen atmosphere containing chalcogen vapor at a partial pressureof the chalcogen greater than or equal to a vapor pressure of thechalcogen at the processing temperature and processing pressure tominimize loss of chalcogen from the precursor layer, wherein theprocessing pressure is a non-vacuum pressure. The suitable atmospheremay comprises of a non-oxygen atmosphere containing chalcogen vapor at apartial pressure of the chalcogen greater than or equal to a vaporpressure of the chalcogen at the processing temperature and processingpressure to minimize loss of chalcogen from the precursor layer, whereinthe processing pressure is a non-vacuum pressure and wherein theparticles are one or more types of binary chalcogenides.

For any of the embodiments described herein, the following may alsoapply. In one embodiment of the present invention, the method comprisesforming a precursor material comprising group IB-chalcogenide and/orgroup IIIA-chalcogenide particles, wherein an overall amount ofchalcogen in the particles relative to an overall amount of chalcogen ina group IB-IIIA-chalcogenide film created from the precursor material,is at a ratio that provides an excess amount of chalcogen in theprecursor material. The method also includes using the precursormaterial to form a precursor layer over a surface of a substrate. Theparticle precursor material is heated in a suitable atmosphere to atemperature sufficient to melt the particles and to release at least theexcess amount of chalcogen from the chalcogenide particles, wherein theexcess amount of chalcogen assumes a liquid form and acts as a flux toimprove intermixing of elements to form the group IB-IIIA-chalcogenidefilm at a desired stoichiometric ratio. The overall amount of chalcogenin the precursor material is an amount greater than or equal to astoichiometric amount found in the IB-IIIA-chalcogenide film.

For any of the embodiments described herein, the following may alsoapply. It should be understood that, optionally, the overall amount ofchalcogen may be greater than a minimum amount necessary to form thefinal IB-IIIA-chalcogenide at the desired stoichiometric ratio. Theoverall amount of chalcogen in the precursor material may be an amountgreater than or equal to the sum of: 1) the stoichiometric amount foundin the IB-IIIA-chalcogenide film and 2) a minimum amount of chalcogennecessary to account for chalcogen lost during processing to form thegroup IB-IIIA-chalcogenide film having the desired stoichiometric ratio.Optionally, the overall amount may be about 2 times greater than aminimum amount necessary to form the IB-IIIA-chalcogenide film at thedesired stoichiometric ratio. The particles may be chalcogen-richparticles and/or selenium-rich particles and/or sulfur-rich particlesand/or tellurium-rich particles. In one embodiment, the overall amountof chalcogen in the group IB-chalcogenide particles is greater than anoverall amount of chalcogen in the group IIIA particles. The overallamount of chalcogen in the group IB-chalcogenide particles may be lessthan an overall amount of chalcogen in the group IIIA particles.

For any of the embodiments described herein, the following may alsoapply. Optionally, the group IB-chalcogenide particles may include a mixof particles, wherein some particles are chalcogen-rich and some arenot, and wherein the chalcogen-rich particles outnumber the particlesthat are not. The group IIIA-chalcogenide particles may include a mix ofparticles, wherein some particles are chalcogen-rich and some are not,and wherein the chalcogen-rich particles outnumber the particles thatare not. The particles may be IBxVIAy and/or IIIAaVIAb particles,wherein x<y and a<b. The resulting group IB-IIIA-chalcogenide film maybe CuzIn(1−x)GaxSe 2, wherein 0.5≦z≦1.5 and 0≦x≦1. The amount ofchalcogen in the particles may be above the stoichiometric ratiorequired to form the film. The particles may be substantiallyoxygen-free particles. The particles may be particles that do notcontain oxygen above about 5.0 weight-percentage. The group IB elementmay be copper. The group IIIA element may be comprised of gallium and/orindium and/or aluminum. The chalcogen may be selenium or sulfur ortellurium. The particles may be alloy particles. The particles may bebinary alloy particles and/or ternary alloy particles and/or multi-naryalloy particles and/or compound particles and/or solid-solutionparticles.

For any of the embodiments described herein, the following may alsoapply. Optionally, the precursor material may include groupIB-chalcogenide particles containing a chalcogenide material in the formof an alloy of a chalcogen and an element of group IB and/or wherein theparticle precursor material includes group IIIA-chalcogenide particlescontaining a chalcogenide material in the form of an alloy of achalcogen and one or more elements of group IIIA. The groupIB-chalcogenide may be comprised of CGS and the group IIIA-chalcogenidemay be comprised of CIS. The method may include adding an additionalsource of chalcogen prior to heating the precursor material. The methodmay include adding an additional source of chalcogen during heating ofthe precursor material. The method may further include adding anadditional source of chalcogen before, simultaneously with, or afterforming the precursor layer. The method may include adding an additionalsource of chalcogen by forming a layer of the additional source over theprecursor layer. The method may include adding an additional source ofchalcogen on the substrate prior to forming the precursor layer. Avacuum-based process may be used to add an additional source ofchalcogen in contact with the precursor layer. The amounts of the groupIB element and amounts of chalcogen in the particles may be selected tobe at a stoichiometric ratio for the group IB chalcogenide that providesa melting temperature less than a highest melting temperature found on aphase diagram for any stoichiometric ratio of elements for the group IBchalcogenide. The method may include using a source of extra chalcogenthat includes particles of an elemental chalcogen. The extra source ofchalcogen may be a chalcogenide. The amounts of the group IIIA elementand amounts of chalcogen in the particles may be selected to be at astoichiometric ratio for the group IIIA chalcogenide that provides amelting temperature less than a highest melting temperature found on aphase diagram for any stoichiometric ratio of elements for the groupIIIA chalcogenide.

For any of the embodiments described herein, the following may alsoapply. Optionally, the group IB-chalcogenide particles may be CuxSey,wherein the values for x and y are selected to create a material with areduced melting temperature as determined by reference to the highestmelting temperature on a phase diagram for Cu—Se. The groupIB-chalcogenide particles may be CuxSey, wherein x is in the range ofabout 2 to about 1 and y is in the range of about 1 to about 2. Thegroup IIIA-chalcogenide particles may be InxSey, wherein the values forx and y are selected to create a material with a reduced meltingtemperature as determined by reference to the highest meltingtemperature on a phase diagram for In—Se. The group IIIA-chalcogenideparticles may be InxSey, wherein x is in the range of about 1 to about 6and y is in the range of about 0 to about 7. The group IIIA-chalcogenideparticles may be GaxSey, wherein the values for x and y are selected tocreate a material with a reduced melting temperature as determined byreference to the highest melting temperature on a phase diagram forGa—Se. The group IIIA-chalcogenide particles may be GaxSey, wherein x isin the range of about 1 to about 2 and y is in the range of about 1 toabout 3. The melting temperature may be at a eutectic temperature forthe material as indicated on the phase diagram. The group IB or IIIAchalcogenide may have a stoichiometric ratio that results in the groupIB or IIIA chalcogenide being less thermodynamically stable than thegroup IB-IIIA-chalcogenide compound.

In yet another embodiment of the present invention, a precursor materialis provided that is comprised of group IB-chalcogenide particlescontaining a substantially oxygen-free chalcogenide material in the formof an alloy of a chalcogen with an element of group IB; and/or groupIIIA-chalcogenide particles containing a substantially oxygen-freechalcogenide material in the form of an alloy of a chalcogen with one ormore elements of group IIIA. The group IB-chalcogenide particles and/orthe group IIIA-chalcogenide particles may have a stoichiometric ratiothat provides a source of surplus chalcogen, wherein the overall amountof chalcogen in the precursor material is an amount greater than orequal to a stoichiometric amount found in the IB-IIIA-chalcogenide film.The overall amount of chalcogen in the precursor material is an amountgreater than or equal to the sum of: 1) the stoichiometric amount foundin the IB-IIIA-chalcogenide film and 2) a minimum amount of chalcogennecessary to account for chalcogen lost during processing to form thegroup IB-IIIA-chalcogenide film having the desired stoichiometric ratio.The overall amount may be greater than a minimum amount necessary toform the IB-IIIA-chalcogenide film at the desired stoichiometric ratio.The overall amount may be about 2 times greater than a minimum amountnecessary to form the IB-IIIA-chalcogenide film at the desiredstoichiometric ratio.

The material may be solid at the processing temperature used indeposition and around the ink preparation temperature complicatingcontrolling particle size. If too much liquid is present at or near theonset of a reaction, liquid may group together at certain areas and notbe evenly distributed throughout the reaction area. This can result inthickness non-uniformity and/or lateral composition non-uniformity.Furthermore, if material in liquid form leaches out from an alloy orcompound containing that material, this may change the localstoichiometry at the start of the reaction resulting in differentcompound(s) in the final thin-film if the leaching occurs prior to orduring processing of the materials. Some embodiments may have acomposition where there is a mixture of elemental Ga and solid Ga4Na.This can be generalized to a composition where there is elemental groupIIIA material and group IA-IIIA material.

A further understanding of the nature and advantages of the inventionwill become apparent by reference to the remaining portions of thespecification and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1D are schematic cross-sectional diagrams illustratingfabrication of a film according to an embodiment of the presentinvention.

FIG. 2 shows the binary phase diagram for a Gallium-Sodium alloy.

FIG. 3 shows the binary phase diagram for an Indium-Sodium alloy.

FIGS. 4A and 4B show particles being processed in accordance with anembodiment of the present invention.

FIG. 5 shows a process flow schematic for creating solid particles usinga dispersion and/or suspension method according to one embodiment of thepresent invention.

FIGS. 6A and 6B show another method of forming solid particles accordingto an embodiment of the present invention.

FIGS. 7A through 7C show one method of bandgap grading according to anembodiment of the present invention.

FIGS. 8A through 8C show another method of bandgap grading according toan embodiment of the present invention.

FIGS. 9A and 9B show another method of bandgap grading according to anembodiment of the present invention.

FIGS. 10A and 10B show yet another method of bandgap grading accordingto an embodiment of the present invention.

FIG. 11 shows the use of spherical particles and non-spherical particlesaccording to an embodiment of the present invention.

FIG. 12 shows the use of spherical particles and planar according to anembodiment of the present invention.

FIGS. 13A through 13E cross-sectional schematics showing that layers ofmaterial may be deposited above and/or below the precursor layeraccording to embodiments of the present invention.

FIG. 14 shows roll-to-roll process according to an embodiment of thepresent invention.

FIG. 15A shows a cross-sectional view of a photovoltaic device accordingto an embodiment of the present invention.

FIG. 15B shows one embodiment of a module according to the presentinvention.

FIG. 16A shows one embodiment of a system for use with rigid substratesaccording to one embodiment of the present invention.

FIG. 16B shows one embodiment of a system for use with rigid substratesaccording to one embodiment of the present invention.

FIGS. 17-19 show the use of inter-metallic material to form a filmaccording to embodiments of the present invention.

FIG. 20 is a cross-sectional view showing the use of multiple layers toform a film according to embodiments of the present invention.

FIG. 21 shows feedstock material being processed according toembodiments of the present invention.

FIGS. 22A and 22B show features of flakes according to embodiments ofthe present invention.

FIGS. 23A and 23B show features of platelets.

FIG. 24 shows the use of spherical inter-metallic material to form afilm according to embodiments of the present invention.

FIG. 25 shows feedstock material being processed according toembodiments of the present invention.

FIG. 26A-26D show cross-sectional view of depositing additionalchalcogen according to one embodiment of the present invention.

FIG. 27A shows a schematic of a system using a chalcogen vaporenvironment according to one embodiment of the present invention.

FIG. 27B shows a schematic of a system using a chalcogen vaporenvironment according to one embodiment of the present invention.

FIG. 27C shows a schematic of a system using a chalcogen vaporenvironment according to one embodiment of the present invention.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the invention, as claimed. It may be notedthat, as used in the specification and the appended claims, the singularforms “a”, “an” and “the” include plural referents unless the contextclearly dictates otherwise. Thus, for example, reference to “a material”may include mixtures of materials, reference to “a compound” may includemultiple compounds, and the like. References cited herein are herebyincorporated by reference in their entirety, except to the extent thatthey conflict with teachings explicitly set forth in this specification.

In this specification and in the claims which follow, reference will bemade to a number of terms which shall be defined to have the followingmeanings:

“Optional” or “optionally” means that the subsequently describedcircumstance may or may not occur, so that the description includesinstances where the circumstance occurs and instances where it does not.For example, if a device optionally contains a feature for a barrierfilm, this means that the barrier film feature may or may not bepresent, and, thus, the description includes both structures wherein adevice possesses the barrier film feature and structures wherein thebarrier film feature is not present.

Photovoltaic Device Chemistry

The solid particles for use with the present invention may be used witha variety of different chemistries to arrive at a desired semiconductorfilm. Although not limited to the following, an active layer for aphotovoltaic device may be fabricated by first formulating an ink ofspherical and/or non-spherical particles each containing at least oneelement from groups IB, IIIA and/or VIA, coating a substrate with theink to form a precursor layer, and heating the precursor layer to form adense film. In a two step process, the dense film may then be processedin a suitable atmosphere to form a group IB-IIIA-VIA compound. In otherembodiments, the precursor layer forms a layer with a group IB-IIIA-VIAcompound in a one step process. Optionally, others may take one or moresteps. It should be understood that reduction and/or densification ofthe precursor layer may not be needed in some embodiments, particularlyif the precursor materials are oxygen-free or substantially oxygen free.Thus, the heating step may optionally be skipped if the particles areprocessed air-free and are oxygen-free. The resulting group IB-IIIA-VIAcompound for either a one step or a two step process may be a compoundof Cu, In, Ga and selenium (Se) and/or sulfur S of the formCuIn_((1-x))Ga_(x)S_(2(1-y))Se_(2y), where 0≦x≦1 and 0≦y≦1. Optionally,it should also be understood that the resulting group IB-IIIA-VIAcompound may be a compound of Cu, In, Ga and selenium (Se) and/or sulfurS of the form Cu_(z)In_((1-x))Ga_(x)S_(2(1-y))Se_(2y), where 0.5≦z≦1.5,0≦x≦1.0 and 0≦y≦1.0. Some embodiments may also form the desiredsemiconductor film in a one step process.

It should be understood that group IB, IIIA, and VIA elements other thanCu, In, Ga, Se, and S may be included in the description of theIB-IIIA-VIA materials described herein, and that the use of a hyphen(“-” e.g., in Cu—Se or Cu—In—Se) does not indicate a compound, butrather indicates a coexisting mixture of the elements joined by thehyphen. It is also understood that group IB is sometimes referred to asgroup 11, group IIIA is sometimes referred to as group 13 and group VIAis sometimes referred to as group 16. Furthermore, elements of group VIA(16) are sometimes referred to as chalcogens. Where several elements canbe combined with or substituted for each other, such as In and Ga, orSe, and S, in embodiments of the present invention, it is not uncommonin this art to include in a set of parentheses those elements that canbe combined or interchanged, such as (In, Ga) or (Se, S). Thedescriptions in this specification sometimes use this convenience.Finally, also for convenience, the elements are discussed with theircommonly accepted chemical symbols. Group IB elements suitable for usein the method of this invention include copper (Cu), silver (Ag), andgold (Au). Preferably the group IB element is copper (Cu). Group IIIAelements suitable for use in the method of this invention includegallium (Ga), indium (In), aluminum (Al), and thallium (Tl). Preferablythe group IIIA element is gallium (Ga) and/or indium (In). Group VIAelements of interest include selenium (Se), sulfur (S), and tellurium(Te), and preferably the group VIA element is either Se and/or S. Itshould be understood that mixtures such as, but not limited to, alloys,solid solutions, and compounds of any of the above can also be used. Theshapes of the solid particles may be any of those described herein.

Method of Forming a Film

Referring now to FIG. 1, one method of forming a semiconductor film fromsolid particles according to the present invention will now bedescribed. It should be understood that the present embodiment of theinvention uses non-vacuum techniques to form the semiconductor film.Other embodiments of the invention, however, may optionally form thefilm under a vacuum environment in one or more steps, and the use ofsolid particles (non-spherical and/or spherical) is not limited to onlynon-vacuum coating techniques.

As seen in FIG. 1, a substrate 102 is provided. By way of non-limitingexample, the substrate 102 may be made of a metal such as aluminum. Inother embodiments, metals such as, but not limited to, stainless steel,molybdenum, titanium, copper, metallized plastic films, coated metalfoils, or combinations of the foregoing may be used as the substrate102. Alternative substrates include but are not limited to ceramics,glasses, and the like. Any of these substrates may be in the form offoils, sheets, rolls, the like, or combinations thereof. Depending onthe material of the substrate 102, it may be useful to form or apply acontact layer 104 to the surface of substrate 102 to promote electricalcontact between the substrate 102 and the absorber layer that is to beformed on it. As a nonlimiting example, when the substrate 102 is madeof aluminum, the contact layer 104 may be but is not limited to a layerof molybdenum. For the purposes of the present discussion, the contactlayer 104 may be regarded as being part of the substrate. As such, anydiscussion of forming or disposing a material or layer of material onthe substrate 102 includes disposing or forming such material or layeron the contact layer 104, if one is used. Optionally, other layers ofmaterials may also be used with the contact layer 104 for insulation orother purposes and still considered part of the substrate 102. It shouldbe understood that the contact layer 104 may comprise of more than onetype or more than one discrete layer of material. Optionally, someembodiments may use any one and/or combinations of the following for thecontact layer: a copper, aluminum, chromium, molybdenum, vanadium, etc.and/or iron-cobalt alloys.

Aluminum and molybdenum can and often do inter-diffuse into one another,especially upon heating to elevated temperatures as used for absorbergrowth, with deleterious electronic and/or optoelectronic effects on thedevice 100. Furthermore aluminum can diffuse though molybdenum intolayers beyond e.g. CIG(S). To inhibit such inter-diffusion, anintermediate, interfacial layer 103 may be incorporated between thealuminum foil substrate 102 and molybdenum base electrode 104. Theinterfacial layer may be composed of any of a variety of materials,including but not limited to chromium, vanadium, tungsten, and glass, orcompounds such as nitrides (including but not limited to titaniumnitride, tantalum nitride, tungsten nitride, hafnium nitride, niobiumnitride, zirconium nitride, vanadium nitride, silicon nitride, ormolybdenum nitride), oxynitrides (including but not limited tooxynitrides of Ti, Ta, V, W, Si, Zr, Nb, Hf, or Mo), oxides, and/orcarbides. The material may be selected to be an electrically conductivematerial. In one embodiment, the materials selected from theaforementioned may be those that are electrically conductive diffusionbarriers. The thickness of this layer can range from 10 nm to 50 nm orfrom 10 nm to 30 nm. Optionally, the thickness may be in the range ofabout 50 nm to about 1000 nm. Optionally, the thickness may be in therange of about 100 nm to about 750 nm. Optionally, the thickness may bein the range of about 100 nm to about 500 nm. Optionally, the thicknessmay be in the range of about 110 nm to about 300 nm. In one embodiment,the thickness of the layer 103 is at least 100 nm or more. In anotherembodiment, the thickness of the layer 103 is at least 150 nm or more.In one embodiment, the thickness of the layer 103 is at least 200 nm ormore. Optionally, some embodiments may include another layer such as butnot limited to a copper layer, a titanium layer, or other metal layerabove the layer 103 and below the base electrode layer 104. Optionally,some embodiments may include another layer such as but not limited to acopper layer, a titanium layer, an aluminum layer, or other metal layerbelow the layer 103 and below the base electrode layer 104. This layermay be thicker than the layer 103. Optionally, it may be the samethickness or thinner than the layer 103. This layer 103 may be placed onone or optionally both sides of the aluminum foil (shown as layer 105 inphantom in FIG. 1).

If barrier layers are on both sides of the aluminum foil, it should beunderstood that the protective layers may be of the same material orthey may optionally be different materials from the aforementionedmaterials. The bottom protective layer 105 may be any of the materials.Optionally, some embodiments may include another layer 107 such as butnot limited to an aluminum layer above the layer 105 and below thealuminum foil 102. This layer 107 may be thicker than the layer 103 (orthe layer 104). Optionally, it may be the same thickness or thinner thanthe layer 103 (or the layer 104). Although not limited to the following,this layer 107 may be comprised of one or more of the following: Mo, Cu,Ag, Al, Ta, Ni, Cr, NiCr, or steel. Some embodiments may optionally havemore than one layer between the protective layer 105 and the aluminumfoil 102. Optionally, the material for the layer 105 may be anelectrically insulating material such as but not limited to an oxide,alumina, or similar materials. For any of the embodiments herein, thelayer 105 may be used with or without the layer 107.

Referring now to FIG. 1B, a precursor layer 106 is formed over thesubstrate 102 by coating the substrate 102 with a dispersion such as butnot limited to an ink. As one nonlimiting example, the ink may becomprised of a carrier liquid mixed with the microflakes 108 and has arheology that allows the ink to be coatable over the substrate 102. Inone embodiment, the present invention may use dry powder mixed with thevehicle and sonicated before coating. Optionally, the inks may bealready formulated as the precursor materials are formed in the mill. Inthe case of mixing a plurality of flake compositions, the product may bemixed from various mills. This mixing could be sonicated but other formsof mechanical agitation and/or another mill may also be used. The inkused to form the precursor layer 106 may contain non-spherical particles108 such as but not limited to microflakes and/or nanoflakes. It shouldalso be understood that the ink may optionally use both non-sphericaland spherical particles in any of a variety of relative proportions.

FIG. 1B includes a close-up view of the microflakes 108 in the precursorlayer 106, as seen in the enlarged image. Microflakes have non-sphericalshapes and are substantially planar on at least one side. A moredetailed view of one embodiment of the microflakes 108 can be found inFIGS. 2A and 2B of U.S. patent application Ser. No. 11/362,266 filedFeb. 23, 2006 and fully incorporated herein by reference. Microflakesmay be defined as particles having at least one substantially planarsurface with a length and/or largest lateral dimension of about 500 nmor more and the particles have an aspect ratio of about 2 or more. Inother embodiments, the microflake is a substantially planar structurewith thickness of between about 10 and about 250 nm and lengths betweenabout 500 nm and about 5 microns. It should be understood that in otherembodiments of the invention, microflakes may have lengths as large as10 microns. Optionally, other microflakes may be those with a thicknessdimension substantially less than its length and width. In such otherembodiments, flakes may be curved or undulating, or other non-planarshape, but still have a high aspect ratio between thickness andlength-to-width. Although not limited to the following, at least some ofthe solid group IIIA-particles may be processed into planar particlesand adapted for use during solution deposition.

It should be understood that different types of particles such asmicroflakes 108 may be used to form the precursor layer 106. In onenonlimiting example, the microflakes are elemental microflakes, i.e.,microflakes having only a single atomic species. The microflakes may besingle metal particles of Cu, Ga, In, or Se. Some inks may have only onetype of microflake. Other inks may have two or more types of microflakeswhich may differ in material composition and/or other quality such asbut not limited to shape, size, interior architecture (e.g. a centralcore surrounded by one or more shell layers) exterior coating, or thelike. In one embodiment, the ink used for precursor layer 106 maycontain microflakes comprising one or more group IB elements andmicroflakes comprising one or more different group IIIA elements.Optionally, the precursor layer 106 contains copper, indium and gallium.In another embodiment, the precursor layer 106 may be an oxygen-freelayer containing copper, indium and gallium. Optionally, the ratio ofelements in the precursor layer may be such that the layer, whenprocessed, forms one or more compounds of a compound ofCuIn_(x)Ga_(1-x), where 0≦x≦1. Those of skill in the art will recognizethat other group IB elements may be partially or completely substitutedfor Cu and other group IIIA elements may be partially or completelysubstituted for In and Ga. Optionally, the precursor may contain Se aswell, such as but not limited to Cu—In—Ga—Se flakes. This is feasible ifthe precursor is oxygen-free and densification is not needed.Optionally, this is also feasible when the precursor layer is notoxygen-free or when densification prior to absorber-growth is desired.Two nonlimiting examples are provided. One nonlimiting example would beto densify a precursor layer that is Se-poor, where the Se is mainlyadded to limit undesired oxidation of the particles, and in a subsequentstep form the absorber layer. Another nonlimiting example would be toform the absorber layer from a Se-poor precursor layer in one stepwithout the need for a separate densification step. In still furtherembodiments, the precursor material may contain microflakes of group IB,IIIA, and VIA elements. In one nonlimiting example, the precursor maycontain Cu—In—Ga—Se microflakes, which would be particularlyadvantageous if the microflakes are formed air free and densificationprior to film formation is not needed.

Optionally, the microflakes 108 in the ink may be of alloy material. Inone nonlimiting example, the alloy microflakes may be binary alloymicroflakes such as but not limited to Cu—In, In—Ga, or Cu—Ga.Alternatively, the microflakes may be a binary alloy of group IB, IIIAelements, a binary alloy of Group IB, VIA elements, and/or a binaryalloy of group IIIA, VIA elements. In other embodiments, the particlesmay be a ternary alloy of group IB, IIIA, and/or VIA elements. Forexample, the particles may be ternary alloy particles of any of theabove elements such as but not limited to Cu—In—Ga. In otherembodiments, the ink may contain particles that are a quaternary alloyof group IB, IIIA, and/or VIA elements. Some embodiments may havequaternary or multi-nary microflakes. The ink may also combinemicroflakes of different classes such as but not limited to elementalmicroflakes with alloy microflakes or the like. In one embodiment of thepresent invention, the microflakes used to form the precursor layer 106contain no oxygen other than those amounts unavoidably present asimpurities. Optionally, the microflakes contain less than about 0.1 wt %of oxygen. In other embodiments, the microflakes contain less than about0.5 wt % of oxygen. In still further embodiments, the microflakescontain less than about 1.0 wt % of oxygen. In yet another embodiment,the microflakes contain less than about 3.0 wt % of oxygen. In otherembodiments, the microflakes contain less than about 5.0 wt % of oxygen.Some embodiments may have a shell layer that contains 0 to 5 wt % ofoxygen. Optionally, the shell may have 5-25 wt % of oxygen. Optionally,the shell may be a full oxide. Any of the foregoing may optionally beapplied to any particles used with the present invention, regardless ofshape or size. It should also be understood that the source of group VIAmaterial may be added as discussed in commonly assigned, co-pending U.S.patent application Ser. No. 11/243,522 (Attorney Docket No. NSL-046)filed on Feb. 23, 2006 and fully incorporated herein by reference.

Optionally, the microflakes 108 in the ink may be chalcogenideparticles, such as but not limited to, a group IB or group IIIAselenide. In one nonlimiting example, the microflakes may be a groupIB-chalcogenide formed with one or more elements of group IB (new-style:group 11), e.g., copper (Cu), silver (Ag), and gold (Au). Examplesinclude, but are not limited to, Cu_(x)Se_(y), wherein x is in the rangeof about 1 to 10 and y is in the range of about 1 to 10. In someembodiments of the present invention, x<y. Alternatively, someembodiments may have selenides that are more selenium rich, such as butnot limited to, Cu₁Se_(x) (where x>1). This may provide an increasedsource of selenium as discussed in commonly assigned, co-pending U.S.patent application Ser. No. 11/243,522 (Attorney Docket No. NSL-046)filed on Feb. 23, 2006 and fully incorporated herein by reference.Alternatively, some embodiments may have selenides that are seleniumpoor, such as but not limited to, Cu₁Se_(x) (where x<1). In anothernonlimiting example, the microflakes may be a group IIIA-chalcogenideformed with one or more elements of group IIIA (new style: group 16),e.g., aluminum (Al), indium (In), gallium (Ga), and thallium (Tl).Examples include In_(x)Se_(y) and Ga_(x)Se_(y) wherein x is in the rangeof about 1 to about 10 and y is in the range of about 1 to about 10.Still further, the microflakes may be a Group IB-IIIA-chalcogenidecompound of one or more group IB elements, one or more group IIIAelements and one or more chalcogens. Examples include CuInGa—Se₂. Otherembodiments may replace the selenide component with another group VIAelement such as but not limited to sulfur, or combinations of multiplegroup VIA elements such as both sulfur and selenium. Any of theforegoing may optionally apply to any particles used with the presentinvention, regardless of shape or size.

It should be understood that the ink used in the present invention mayinclude more than one type of chalcogenide microflakes. For example,some may include microflakes from both group IB-chalcogenide(s) andgroup IIIA-chalcogenide(s). Others may include microflakes fromdifferent group IB-chalcogenides with different stoichiometric ratios.Others may include microflakes from different group IIIA-chalcogenideswith different stoichiometric ratios.

Optionally, the microflakes 108 in the ink may also be particles of atleast one solid solution. In one nonlimiting example, the nano-powdermay contain copper-gallium solid solution particles, and at least one ofindium particles, indium-gallium solid-solution particles, copper-indiumsolid solution particles, and copper particles. Alternatively, thenano-powder may contain copper particles and indium-galliumsolid-solution particles. Yet in another nonlimiting example, thenano-powder may contain both copper particles and copper-indium-galliumsolid-solution particles

Generally, an ink may be formed by dispersing the microflakes (and/orother particles) in a vehicle containing a dispersant (e.g., asurfactant or polymer) along with (optionally) some combination of othercomponents commonly used in making inks. In some embodiments of thepresent invention, the ink is formulated without a dispersant or otheradditives. The carrier liquid may be an aqueous (water-based) ornon-aqueous (organic) solvent. Other components include, withoutlimitation, dispersing agents, binders, emulsifiers, anti-foamingagents, dryers, solvents, fillers, extenders, thickening agents, filmconditioners, anti-oxidants, flow and leveling agents, plasticizers andpreservatives. These components can be added in various combinations toimprove the film quality and optimize the coating properties of themicroflake dispersion. An alternative method to mixing microflakes andsubsequently preparing a dispersion from these mixed microflakes wouldbe to prepare separate dispersions for each individual type ofmicroflake and subsequently mixing these dispersions. It should beunderstood that, due to favorable interaction of the planar shape of themicroflakes with the carrier liquid, some embodiments of the ink may beformulated by use of a carrier liquid and without a dispersing agent.

The precursor layer 106 from the dispersion may be formed on thesubstrate 102 by any of a variety of solution-based coating techniquesincluding but not limited to wet coating, spray coating, spin coating,doctor blade coating, contact printing, top feed reverse printing,bottom feed reverse printing, nozzle feed reverse printing, gravureprinting, microgravure printing, reverse microgravure printing, commadirect printing, roller coating, slot die coating, meyerbar coating, lipdirect coating, dual lip direct coating, capillary coating, ink-jetprinting, jet deposition, spray deposition, and the like, as well ascombinations of the above and/or related technologies. The foregoing mayapply to any embodiments herein, regardless of particle size or shape.

In some embodiments, extra chalcogen, alloys particles, or elementalparticles, e.g., micron- or sub-micron-sized chalcogen powder may bemixed into the dispersion containing the microflakes so that themicroflakes and extra chalcogen are deposited at the same time.Alternatively the chalcogen powder may be deposited on the substrate ina separate solution-based coating step before or after depositing thedispersion containing the particles. In other embodiment, group IIIAelemental material such as but not limited to gallium droplets may bemixed with the flakes. This is more fully described in commonlyassigned, copending U.S. patent application Ser. No. 11/243,522(Attorney Docket No. NSL-046) filed on Feb. 23, 2006 and fullyincorporated herein by reference. This may create an additional layer107 (shown in phantom in FIG. 1C). Optionally, additional chalcogen maybe added by any combination of (1) any chalcogen source that can besolution-deposited, e.g. a Se, S, or calcogen-containing alloy nano- ormicron-sized powder mixed into the precursor layers or deposited as aseparate layer, (2) chalcogen (e.g., Se or S) evaporation, (3) an H₂Se(H₂S) atmosphere, (4) a chalcogen (e.g., Se or S) atmosphere, (5) an H₂atmosphere, (6) an organo-selenium atmosphere, e.g. diethylselenide oranother organo-metallic material, (7) another reducing atmosphere, e.g.CO, (8) a wet chemical reduction step, and a (9) heat treatment. Thestoichiometric ratio of microflakes to extra chalcogen, given asSe/(Cu+In+Ga+Se) may be in the range of about 0 to about 1000. This ispurely exemplary and nonlimiting. Any of the foregoing may optionallyapply to any particles used with the present invention, regardless ofshape or size.

Note that the solution-based deposition of the proposed mixtures ofmicroflakes does not necessarily have to be performed by depositingthese mixtures in a single step. In some embodiments of the presentinvention, the coating step may be performed by sequentially depositingmicroflake dispersions having different compositions of IB-, IIIA- andchalcogen-based particulates in two or more steps. For example, themethod may be to first deposit a dispersion containing an indiumselenide microflake (e.g. with an In-to-Se ratio of ˜1), andsubsequently deposit a dispersion of a copper selenide microflake (e.g.with a Cu-to-Se ratio of ˜1) and a gallium selenide microflake (e.g.with a Ga-to-Se ratio of ˜1) followed optionally by depositing adispersion of Se. This would result in a stack of three solution-baseddeposited layers, which may be heated together. Alternatively, eachlayer may be heated before depositing the next layer. A number ofdifferent sequences are possible. For example, a layer ofIn_(x)Ga_(y)Se_(z) with x≧0 (larger than or equal to zero), y≧0 (largerthan or equal to zero), and z≧0 (larger than or equal to zero), may beformed as described above on top of a uniform, dense layer ofCu_(w)In_(x)Ga_(y) with w≧0 (larger than or equal to zero), x≧0 (largerthan or equal to zero), and y≧0 (larger than or equal to zero), andsubsequently converting the two layers into CIGS. Alternatively a layerof Cu_(w)In_(x)Ga_(y) may be formed on top of a uniform, dense layer ofIn_(x)Ga_(y)Se_(z) and subsequently converting the two layers into CIGS.

In alternative embodiments, microflake-based dispersions as describedabove may further include elemental IB, and/or IIIA nanoparticles (e.g.,in metallic form). These nanoparticles may be in flake form, oroptionally, take other shapes such as but not limited to spherical,spheroidal, oblong, cubic, or other non-planar shapes. These particlesmay also include but is not limited to emulsions, molten materials,mixtures, and the like, in addition to solids. For exampleCu_(x)In_(y)Ga_(z)Se_(u) materials, with u>0 (larger than zero), withx≧0 (larger than or equal to zero), y≧0 (larger than or equal to zero),and z≧0 (larger than or equal to zero), may be combined with anadditional source of selenium (or other chalcogen) and metallic galliuminto a dispersion that is formed into a film on the substrate byheating. Metallic gallium nanoparticles and/or nanoglobules and/ornanodroplets may be formed, e.g., by initially creating an emulsion ofliquid gallium in a solution. Gallium metal or gallium metal in asolvent with or without emulsifier may be heated to liquefy the metal,which is then sonicated and/or otherwise mechanically agitated in thepresence of a solvent. Agitation may be carried out either mechanically,electromagnetically, or acoustically in the presence of a solvent withor without a surfactant, dispersant, and/or emulsifier. The galliumnanoglobules and/or nanodroplets can then be manipulated in the form ofa solid-particulate, by quenching in an environment either at or belowroom temperature to convert the liquid gallium nanoglobules into solidgallium nanoparticles. This technique is described in detail incommonly-assigned U.S. patent application Ser. No. 11/081,163 to MatthewR. Robinson and Martin R. Roscheisen entitled “Metallic Dispersion”, theentire disclosures of which are incorporated herein by reference.

Referring now to FIG. 1C, the precursor layer 106 may then be processedin a suitable atmosphere to form a film. The film may be a dense film.In one embodiment, this involves heating the precursor layer 106 to atemperature sufficient to convert the ink (as-deposited ink. Note thatsolvent and possibly dispersant have been removed by drying) to a film.The temperature may be between about 375° C. and about 525° C. (a safetemperature range for processing on aluminum foil orhigh-melting-temperature polymer substrates). The processing may occurat various temperatures in the range, such as but not limited to 450° C.In other embodiments, the temperature at the substrate may be betweenabout 400° C. and about 600° C. at the level of the precursor layer, butoptionally cooler at the substrate. The time duration of the processingmay also be reduced by at least about 20% if certain steps are removed.In one embodiment, the heating may occur over a range between about twominutes to about ten minutes. In one embodiment, the processingcomprises heating the precursor layer to a temperature greater thanabout 375° C. but less than a melting temperature of the substrate for aperiod of less than about 15 minutes. In another embodiment, theprocessing comprises heating the precursor layer to a temperaturegreater than about 375° C. but less than a melting temperature of thesubstrate for a period of about 1 minute or less. In a still furtherembodiment, the processing comprises heating the precursor layer to anannealing temperature but less than a melting temperature of thesubstrate for a period of about 1 minute or less. The processing stepmay also be heated and/or accelerated via thermal processing techniquesusing at least one of the following processes: pulsed thermalprocessing, exposure to laser beams, or heating via IR lamps, and/orsimilar or related processes. Although not limited by the following,one-step processes typically occur in a reactive atmosphere, whilemulti-step processes may include one or steps in non-reactiveatmosphere(s) while the remaining steps are in an reactive atmosphere.

Although pulsed thermal processing remains generally promising, certainimplementations of the pulsed thermal processing such as a directedplasma arc system, face numerous challenges. In this particular example,a directed plasma arc system sufficient to provide pulsed thermalprocessing is an inherently cumbersome system with high operationalcosts. The direct plasma arc system requires power at a level that makesthe entire system energetically expensive and adds significant cost tothe manufacturing process. The directed plasma arc also exhibits longlag time between pulses and thus makes the system difficult to mate andsynchronize with a continuous, roll-to-roll system. The time it takesfor such a system to recharge between pulses also creates a very slowsystem or one that uses more than one directed plasma arc, which rapidlyincrease system costs.

In some embodiments of the present invention, other devices suitable forrapid thermal processing may be used and they include pulsed lasers usedin adiabatic mode for annealing (Shtyrokov E I, Sov. Phys.—Semicond. 91309), continuous wave lasers (10-30 W typically) (Ferris S D 1979Laser-Solid Interactions and Laser Processing (New York: AIP)), pulsedelectron beam devices (Kamins T 11979 Appl. Phys. Leti. 35 282-5),scanning electron beam systems (McMahon R A 1979 J. Vac. Sci. Techno. 161840-2) (Regolini J L 1979 Appl. Phys. Lett. 34 410), other beam systems(Hodgson R T 1980 Appl. Phys. Lett. 37 187-9), graphite plate heaters(Fan J C C 1983 Mater. Res. Soc. Proc. 4 751-8) (M W Geis 1980 Appl.Phys. Lett. 37 454), lamp systems (Cohen R L 1978 Appl. Phys. Lett. 33751-3), and scanned hydrogen flame systems (Downey D F 1982 Solid StateTechnol. 25 87-93). In some embodiments of the present invention, anon-directed, low density system may be used. Alternatively, other knownpulsed heating processes are also described in U.S. Pat. Nos. 4,350,537and 4,356,384. Additionally, it should be understood that methods andapparatus involving pulsed electron beam processing and rapid thermalprocessing of solar cells as described in expired U.S. Pat. Nos.3,950,187 (“Method and apparatus involving pulsed electron beamprocessing of semiconductor devices”) and 4,082,958 (“Apparatusinvolving pulsed electron beam processing of semiconductor devices”) arein the public domain and well known. U.S. Pat. No. 4,729,962 alsodescribes another known method for rapid thermal processing of solarcells. The above may be applied singly or in single or multiplecombinations with the above or other similar processing techniques withvarious embodiments of the present invention.

It should be noted that using microflakes typically results in precursorlayers that heat into a solid layer at temperatures as much as 50° C.lower than a corresponding layer of spherical nanoparticles. This is duein part because of the greater surface area contact between particles.Of course, it should be understood that the use of solid groupIIIA-based particles is not limited to only planar particles such asmicroflakes, and those solid group IIIA-based particles may be suitablefor particles of various shapes.

In certain embodiments of the invention, the precursor layer 106 (or anyof its sub-layers) may be annealed, either sequentially orsimultaneously. Such annealing may be accomplished by rapid heating ofthe substrate 102 and precursor layer 106 from an ambient temperature toa plateau temperature range of between about 200° C. and about 600° C.In this embodiment of the invention, the temperature is maintained inthe plateau range for a period of time ranging between about a fractionof a second to about 60 minutes, and subsequently reduced.Alternatively, the annealing temperature could be modulated to oscillatewithin a temperature range without being maintained at a particularplateau temperature. This technique (referred to herein as rapid thermalannealing or RTA) is particularly suitable for forming photovoltaicactive layers (sometimes called “absorber” layers) on metal foilsubstrates, such as but not limited to aluminum foil. Other suitablesubstrates include but are not limited to other metals such as StainlessSteel, Copper, Titanium, or Molybdenum, metallized plastic foils, glass,ceramic films, and mixtures, alloys, and blends of these and similar orrelated materials. The substrate may be flexible, such as the form of afoil, or rigid, such as the form of a plate, or combinations of theseforms. Additional details of this technique are described in U.S. patentapplication Ser. No. 10/943,685, which is incorporated herein byreference.

The atmosphere associated with the annealing step may also be varied. Inone embodiment, the suitable atmosphere comprises a hydrogen atmosphere.In another embodiment the suitable atmosphere comprises a carbonmonoxide atmosphere. However, in other embodiments where very low or noamounts of oxygen are found in the microflakes and/or other particles,the suitable atmosphere may be comprised of a nitrogen atmosphere, anargon atmosphere, a carbon monoxide atmosphere, or an atmosphere havingless than about 10% hydrogen. These other atmospheres may beadvantageous to enable and improve material handling during production.

Referring now to FIG. 1D, the precursor layer 106 is processed to formthe dense film 110. The dense film 110 may actually have a reducedthickness compared to the thickness of the wet precursor layer 106 sincethe carrier liquid and other materials have been removed duringprocessing. In one embodiment, the film 110 may have a thickness in therange of about 0.5 microns to about 2.5 microns. In other embodiments,the thickness of film 110 may be between about 1.5 microns and about2.25 microns. In one embodiment, the resulting dense film 110 may besubstantially void free. In some embodiments, the dense film 110 has avoid volume of about 5% or less. In other embodiments, the void volumeis about 10% or less. In another embodiment, the void, volume is about20% or less. In still other embodiments, the void volume is about 24% orless. In still other embodiments, the void volume is about 30% or less.The processing of the precursor layer 106 will fuse the microflakestogether and in most instances, remove void space and thus reduce thethickness of the resulting dense film.

Depending on the type of materials used to form the film 110, the film110 may be a film for use in a one step process, or a two step process,or a multi-step process. In a one step process, the film 110 is formedto include group IB-IIIA-VIA compounds and the film 110 may be anabsorber film suitable for use in a photovoltaic device. In a two stepprocess, the film 110 may be a solid and/or densified film that willhave further processing to be suitable for use as an absorber film foruse in a photovoltaic device. As a nonlimiting example, the film 110 ina two step process may not contain any and/or sufficient amounts of agroup VIA element to function as an absorber layer. Adding a group VIAelement or other material may be the second step of the two-stepprocess. Either a mixture of two or more VIA elements can be used, or athird step can be added with another VIA element as used in the secondstep. A variety of methods of adding that material include printing ofgroup VIA element, using VIA element vapor, and/or other techniques. Itshould also be understood that in a two step process, the processatmospheres may be different. By way of nonlimiting example, oneatmosphere may optionally be a group VIA-based atmosphere. As anothernonlimiting example, one atmosphere may be an inert atmosphere asdescribed herein. It should be understood that this further processingmay actually react the densified film into a layer with increasedthickness.

Optionally, the present invention may comprise of adding a material tosolidify micron-sized or larger feedstock (to be used to preparesub-micron or nano-sized particles), that otherwise would be all orpartially liquid at particle preparation, handling, or deposition orpre-deposition temperature. In another embodiment, the present inventionmay comprise of adding material to solidify sub-micron or nano-sizedglobules/droplets, that otherwise would be all or partially liquid atparticle preparation, handling, or deposition or pre-depositiontemperature. All combinations of size (large feedstock and sub-micron),process temperature (particle preparation, ink and web handling, anddeposition), and timing (before size reduction, after size reduction)are considered herein.

Solid Group IIIA Particles

Referring now to FIG. 2, various methods of forming the solid particlessuch as but not limited to solid group IIIA particles will now bedescribed. For some of the embodiments described herein, it may bedesirable to have particles that are in solid form. This may beparticularly useful for processing group IIIA-based materials inpreparation for introduction into the precursor layer and/or resultingsemiconductor film. Solid particles may allow for the use of knownprocessing techniques for size reduction and/or shape alteration on thegroup IIIA-based material prior to dispersing the material in a carrierliquid. This may simplify processing and increase process robustness.

Obtaining solid particles is generally not an issue for many of theelements used in forming CIGS based solar cells because they are solidat/or near room temperature. However, some materials, particularly groupIIIA-based materials, may be in liquid form at/or near room temperature,and the liquid form may increase the difficulty of handling the materialor reducing the material to sufficiently small particle and/or dropletsizes. For example, elemental gallium (Ga) may be liquid at temperatureshigher than 30° C., and elemental indium (In) may be liquid attemperatures above 156° C. Certainly, there are many possible ways forincluding these Group IIIA elements into printed CIGS solar cellsincluding, but not limited to using a liquid metallic dispersion ofliquid group IIIA elements. One such method is described in commonlyassigned copending U.S. patent application Ser. No. 11/081,163, filedMar. 16, 2005 and in copending U.S. patent application Ser. No.10/782,017, filed Feb. 19, 2004, both fully incorporated herein byreference for all purposes.

It is possible, however, to solidify Group IIIA based materials andincrease their melting temperatures. This may advantageously increasethe robustness of the thin-film manufacturing process. A variety ofmaterials may be introduced in appropriate amounts to change thecharacteristics of elemental gallium or indium and create solidparticles of Group IIIA-based materials. The resulting solid materialsmay be, but are not limited to, metallic alloys, chalcogen-based alloys,and/or salts. In one embodiment of the present invention, sodium may bethe material introduced to increase the melting temperature of theresulting alloy. Advantageously, sodium is not a contaminant that needsto be removed from the resulting Group IB-IIIA-VIA film. Concurrently,sodium may improve the performance of the photovoltaic device.Furthermore, an alloy of a Group IIIA element and an added material suchas, but not limited to, sodium or other group IA elements will be insolid state well above room temperature and above all sizereduction/shape altering/particle formation processes used with thematerials. This allows spherical and/or non-spherical particles to bemade via processes such as but not limited to milling, evaporationcondensation (EC), electroexplosive wire (EEW), plasma pulse processing,or other methods to create particles desired for use in the presentinvention.

Referring now to FIG. 2, a phase diagram of gallium and sodium (Ga—Na)is shown. As seen in FIG. 2, the phase diagram of Ga—Na indicates thatthe melting point of the binary alloy steady rises from about 30° C. toabout 499° C. as the weight percent of sodium is increased from about 0%to about 8% the amount of solid material increases and stays solid athigher temperature compared to pure gallium (30° C.), up to about 499°C. at about 8%. (as seen along the top axis). Hence, the addition ofsodium will substantially increase the range of temperatures where atleast part of the Ga—Na is a solid and can be handled or sized-reducedwhile in solid form. At about 7.6 weight percent, the alloy may bestable as Ga₄Na), the Ga—Na alloy turns into a line-compound and is allsolid up to 499° C. Compounds containing lower amounts of sodium maycontain portions that separate out into elemental Gallium. At higherweight percentages such as about 15.7% Na, the alloy may be stable asGa₃₉Na₂₂. The melting temperature of such an alloy of Ga₃₉Na₂₂ may be ashigh as about 556° C. This is substantially higher than the handling andprocessing temperature associated with preparing the particles fordeposition and is one method of introducing Gallium using a stable,solid particle.

Referring now to FIG. 3, a similar result can also be found for othergroup IIIA based alloys such as indium and sodium (In—Na). At about 11.1weight percent of sodium, the In—Na alloy turns into a line-compound, isIn₈Na₅ and may have a melting temperature as high as 441° C. At about16.7 weight percent of sodium, the In—Na alloy turns into theline-compound is InNa and may have a melting temperature as high as 345°C. Again, the addition of a second material, which in this case issodium to indium, increases the range of temperatures where the groupIIIA-based particle is a solid particle and can be handled and processedin the same manner as other solid particles.

As a nonlimiting example, the range of materials suitable for use inincreasing the temperature where liquid is formed Accordingly, as seenwith regards to FIGS. 2 and 3, material such as a Group IA element maybe added to a Group IIIA element to solidify the Group IIIA element thatotherwise would be all or partially liquid at particle preparation,handling, or deposition or pre-deposition temperature. This material maybe added to micron-sized or larger Group IIIA feedstock to be used toprepare sub-micron or nano-sized particles. Optionally, the material maybe added to solidify sub-micron or nano-sized globules/droplets thatotherwise would be all or partially liquid at particle preparation,handling, or deposition or pre-deposition temperature. The amount ofgroup IA material may be adjusted to account for any combinations ofsize (large feedstock and sub-micron), process temperature (particlepreparation, ink and web handling, and deposition), and timing (beforesize reduction, after size reduction) to reduce premature presence ofliquid.

In addition to Group IA elements mentioned above, another embodiment ofthe invention may use other materials that can maintain a substantiallyall solid material. As a nonlimiting example, the range of materialsuitable for use in increasing the melting and/or reacting temperatureof a group IIIA-based material may include: sodium, lithium, potassium,rubidium, cesium, sulfur, selenium, rare-earth elements, and/oraluminum. This suitable material may include one or more group IA-basedmaterial (in elemental, alloy, or compound form). By way of example andnot detract from semiconductor film quality include, but are not limitedto: lithium, potassium, rubidium, cesium, sulfur, aluminum, and/orcombinations thereof.

As a nonlimiting example, the range of materials suitable for use inincreasing the temperature where liquid is formed in a group IIIA-basedmaterial may include one or more group IA-based materials (in elemental,alloy, or compound form). By way of example and without limitation,Table I shows some of the possible combinations.

TABLE I Na Li K Rb In In—Na In—Li In—K In—Rb Ga Ga—Na Ga—Li Ga—K Ga—RbIn—Ga In—Ga—Na In—Ga—Li In—Ga—K In—Ga—Rb Cu—In—Ga Cu—In—Ga—NaCu—In—Ga—Li Cu—In—Ga—K Cu—In—Ga—Rb Al—Ga Al—Ga—Na Al—Ga—Li Al—Ga—KAl—Ga—Rb Al—In Al—In—Na Al—In—Li Al—In—K Al—In—Rb Cu—Al—Ga Cu—Al—Ga—NaCu—Al—Ga—Li Cu—Al—Ga—K Cu—Al—Ga—Rb Cu—Al—In Cu—Al—In—Na Cu—Al—In—LiCu—Al—In—K Cu—Al—In—Rb Cu—In—Se Cu—In—Se—Na Cu—In—Se—Li Cu—In—Se—KCu—In—Se—Rb Cu—Ga—Se Cu—Ga—Se—Na Cu—Ga—Se—Li Cu—Ga—Se—K Cu—Ga—Se—RbIn—Ga—Se In—Ga—Se—Na In—Ga—Se—Li In—Ga—Se—K In—Ga—Se—Rb Cu—In—Ga—SeCu—In—Ga—Se—Na Cu—In—Ga—Se—Li Cu—In—Ga—Se—K Cu—In—Ga—Se—Rb Cs S Al InIn—Cs In—S In—Al Ga Ga—Cs Ga—S Ga—Al In—Ga In—Ga—Cs In—Ga—S In—Ga—AlCu—In—Ga Cu—In—Ga—Cs Cu—In—Ga—S Cu—In—Ga—Al Al—Ga Al—Ga—Cs Al—Ga—S Al—GaAl—In Al—In—Cs Al—In—S Al—In Cu—Al—Ga Cu—Al—Ga—Cs Cu—Al—Ga—S Cu—Al—GaCu—Al—In Cu—Al—In—Cs Cu—Al—In—S Cu—Al—In Cu—In—Se Cu—In—Se—Cs Cu—In—Se—SCu—In—Se—Al Cu—Ga—Se Cu—Ga—Se—Cs Cu—Ga—Se—S Cu—Ga—Se—Al In—Ga—SeIn—Ga—Se—Cs In—Ga—Se—S In—Ga—Se—Al Cu—In—Ga—Se Cu—In—Ga—Se—CsCu—In—Ga—Se—S Cu—In—Ga—Se—Al

As a nonlimiting example, the range of materials suitable for use inincreasing the temperature where liquid is formed in a group IIIA-basedmaterial may include one or more group IA-based materials (in elemental,alloy, or compound form). By way of example and without limitation,Table I shows some of the possible combinations. Various sodium saltsand other salt compounds may added to Gallium or other group IIIAelements to form solid compounds. Although not limited to the following,some examples of Gallium-based compounds include: Ga—Na—F (better leaveout stoichiometry), GaF₃, and or Ga(NO₃)₃. Similar Indium basedcompounds may also be used. Basically, any Ga, In, or Ga—In-salt couldbe included, e.g. any halide as counter-anion (although Cl less optimalas it may decrease performance of CIGS cells), sulfates, sulfites,nitrates, phosphates, hydroxides, selenites, borates, acetate, butyrate,hexanoate, etc. . . . Although not limited to the following, the saltsmay be selected to NOT be soluble in the solvent. The salt counter-ionmay easily be decomposed with the counterion decomposing into volatiles,either by heating in an inert atmosphere, heating in a reducingatmosphere, heating in a selenizing (sulfurizing) atmosphere, or anycombination of the previous. Additionally, any other conceivable methodof replacing the counter-ion by Se and/or S (e.g. wet chemically) wouldallow counter-ions that do not decompose under heat, H2, and/or aselenizing or sulfurizing atmosphere.

Apart from alloys of IIIA and sodium, sodium can be added in differentways as well. Other suitable sodium containing compounds include anydeprotonated organic and inorganic acid, deprotonated alcohol where theproton is replaced by sodium. The list may also include deprotonatedacids, being the sodium salt of the (deprotonated) acid, sodiumhydroxide, sodium acetate, and the sodium salts of e.g. the followingacids: Butyric acid, Caproic Acid, Caprylic Acid, Capric Acid, LaurieAcid, Myristic Acid, Palmitic Acid, Palmitoleic Acid, Stearic Acid,Oleic Acid, Vaccenic Acid, Linoleic Acid, Alpha-Linolenic Acid,Gamma-Linolenic Acid. Other possibilities include deprotonated alcoholssuch as sodium ethoxide. Other inorganic compounds include sodiumnitrate, sodium selenite, sodium sulphate, sodium sulphite, sodiumphosphate, and/or sodium phospite.

In another embodiment of the present invention, the technique of using agroup IIIA-based alloy to introduce a group IIIA element into thesemiconductor film may be optimized if the material alloyed with thegroup IIIA element is a material that does not need to be subsequentlyremoved from the semiconductor film. Sodium may be advantageous in thisregard. Other materials that may be used in amounts that will notdetract from semiconductor film quality include, but are not limited to:sodium, lithium, potassium, rubidium, cesium, sulfur, aluminum, and/orcombinations thereof. Materials containing high amounts of carbon (C),nitrogen (N), or oxygen (O), or fluoride (F) would leave residuals thatmay need to be removed to maximize performance of the resultingsemiconductor film.

As further example of solid Group IIIA-based materials, solid Gaparticles can optionally also be created via temperature control (Ga<29°C.) or when combined to form Cu—Ga, Cu—Ga—n, Ga—Se, Ga—S, In—Ga—S,In—Ga—Se, etc., Ga-IA (e.g. with Group IA e.g. Na, K, Li), Ga-salts(e.g. GaF₃, Ga(NO₃)₃). For certain embodiments of the present inventionusing salts and even for more exotic organo-gallium compounds, theelement and/or materials added to Ga are preferably removed prior to,during, or after the formation of CIGS to minimize the amount of C, N,O, F, etc. in the CIGS film as previously mentioned.

The alloy may be formed by a variety of methods such as, but not limitedto, atomization, pyrometallurgy, mechanical alloying, or combinationsthereof. Bulk materials of the alloy may be treated by the following toform particles using at least one of the following methods: grinding,milling, electroexplosive wire (EEW) processing, evaporationcondensation (EC), pulsed plasma processing, or combinations thereof.Optionally, the particles may be formed using at least one of thefollowing methods: spray-pyrolysis, laser pyrolysis, or a bottom-uptechnique like wet chemical approaches. It should be understood that insome embodiments, further processing may be used to refine the materialcreated as described above. For example, mechanical alloying may be usedto combine a material such as Ga—Na with Cu—In or other materials. Thismay be particularly useful if a ternary or multi-nary alloy is too hardto mill into smaller pieces or different shapes. In some embodiments,instead of starting with an atomically homogeneously mixed feedstock, amixture of two or more start materials each having a differentcomposition may be using during mechanical milling.

The particles created above may be used in a precursor material in avariety of substances including a solution coatable ink for forming afilm on a substrate. The method may include formulating an inkcontaining the precursor material and then solution depositing the inkonto a substrate to form a precursor layer on the substrate. Of courseas previously described, the precursor layer may be reacted in asuitable atmosphere to form a group IB-IIIA-VIA based film in a one stepprocess or it may become a group IB-IIIA-VIA based film via a two-stepor multi-step process.

The solid IIIA-based particles may optionally be a sole source of groupIIIA elements in the ink. In terms of composition, the resulting filmmay have a Cu/(In+Ga) compositional range of about 0.01 to about 1.0 anda Ga/(In+Ga) compositional range of about 0.01 to about 1.0. The filmmay have a Cu/(In+Ga) compositional range of about >1.0 for Cu/(In+Ga)and a Ga/(In+Ga) compositional range of about 0.01 to about 1.0. Thefilm may have a Cu/(In+Ga) compositional range of about 0.01 to about1.0 and a Ga/(In+Ga) compositional range of about 0.01 to about 1.0.Optionally, the film has a desired Cu/(In+Ga) molar ratio is in therange of about 0.7 to about 1.0 and a desired Ga/(Ga+In) molar ratio inthe range of about 0.1 to about 0.8. Optionally, there is thepossibility of having a ratio of Cu/(In+Ga)>1.0 and using subsequentpost-treatment (KCN, etc.) to change Cu/(In+Ga)<1.0.

Adding Material to Solidify an Alloy

Referring now to FIGS. 4A and 4B, another embodiment of the presentinvention adds a material 130 such as but not limited to sodium to metalalloys 132 that might ordinarily be characterized by leaching or phaseseparation of a component element from the alloy particle containsregions that are liquid and other regions that are solid (i.e. phaseseparation of a group IIIA component such as liquid gallium). In onesuch example involving a group IIIA-based alloy, a group IIIA alloy suchas copper gallium may have gallium leach from the alloy as thetemperature increases and/or during mechanical alloying. This leachingis generally undesirable since this may change the stoichiometry of theresulting reaction if the leached material is lost and the leechingoccurs prior to processing of the materials. It may also bedisadvantageous during processing since the liquid form may change thekinetics of the reaction. For example, if too much liquid is present ator near the onset of a reaction, liquid may group together at certainareas and not be evenly distributed throughout the reaction area. Inother situations, the leached material may gum-up or clog equipment usedin milling.

To solidify elements which may leach in liquid form at particlepreparation, handling, or deposition or pre-deposition temperature, amaterial as described previously may be combined with the alloy. Thismay include adding a group IA element or IA-based material such assodium to metal alloys such as In—Ga, Cu—In—Ga, In—Ga—Se, Ga—Se andother metal alloys as described in Table I. The methods used to makethis solid alloy 134 may include any of those described previouslyherein. The addition of this material overcomes gallium leaching whichmay occur during milling and/or mechanical alloying of the bulkmaterials.

It should be understood that a binary or multinary alloy (such as butnot limited to IB-IIIA-VIA-IA) in a broad range of compositions hardlyever consists of one (solid) line-compound, but typically is acombination of two or more line-compounds and solid-solutions, where oneor more compounds might be present as a liquid at processingtemperature. As a non-limiting example, Ga—Na at 300° C. with acomposition of 15 atomic percent Na consists mainly of solid Ga₄Na (atthermodynamic equilibrium, see phase diagram) and some, almost pure,liquid gallium. Another nonlimiting example, Ga—Na at 200° C. with acomposition of 28 atomic percent sodium consists of a mixture of twodifferent solids, being Ga₄Na and Ga₃₉Na₂₂. In other words, a materialwith a formula like e.g. Cu—In—Ga (with or without details on the actualstoichiometry of the bulk material) might consist of one, but morecommonly, a mixture of two or more different compounds of differentcompositions.

The material 130 to be added to the alloy may be prepared using avariety of methods such as but not limited to atomization,pyrometallurgy, mechanical alloying, or combinations thereof. Theresulting material may then be treated using the previously mentionedmethods of grinding, milling, electroexplosive wire (EEW) processing,evaporation condensation (EC), pulsed plasma processing, or combinationsthereof.

FIG. 4B shows that the particles need not be spherical and may have amaterial represented in a non-spherical particle 140 and the alloyrepresented by non-spherical particle 142. The resulting alloy may alsobe non-spherical particles 144. Of course, combinations of spherical andnon-spherical particles are also combinable. It should also beunderstood that the group IIIA elements may be introduced where one ormore are introduced as a liquid (or as liquid droplets/nanoglobules) incombination with solid particles.

The amount of material added may be in trace or dopant amounts or it maybe in sufficient amounts to alter the composition of the resulting film.In one embodiment, the final film may include 1) a group IB-IIIA-VIA-IAcompound and/or 2) at least a mixture of one or more IB-IIIA-VIAcompounds and one or more IA-containing compounds. Of course, otherembodiments may use other materials such as but not limited to rareearth elements in place of IA material. Because the atomic concentrationof group IA material may be much lower than the concentration of the IB,IIIA, and VIA elements, the IA elements are typically not mentioned andare seen as dopants.

Quenching Group IIIA Droplets Via Addition of Group IA Material

Referring now to FIG. 5, it should be understood that the formation ofsolid group IIIA-based alloy particles may be achieved by a variety ofmethods many of which were discussed above. The present embodimentdescribes a still further process that uses an emulsion-based processwherein a material such as sodium is added to an emulsion of a groupIIIA-based material to solidify droplets and/or nanoparticles in theemulsion. The formed solid material may be Ga₄Na or other similarmaterials that are solid at or near room temperature. The sodium may beadded in the form of elemental sodium and/or a sodium-based compound,either to a group IIIA-based emulsion (of droplets) or dispersion (ofsolid particles). Other embodiments may add liquid sodium (melting point100° C.), either to a group IIIA-based emulsion (of droplets) ordispersion (of solid particles). Still further embodiments may add asodium-based emulsion, either to a group IIIA-based emulsion (ofdroplets), or to a group IIIA-based dispersion (of solid particles).).Still further embodiments may add a sodium-based emulsion to agallium-based emulsion.

As seen in the example of FIG. 5, an emulsion of gallium is created byadding gallium shot 150 into a carrier liquid 152. The carrier liquid152 may be a solvent and/or include a surfactant. Optionally, thecarrier liquid 152 may be a non-organic solvent. In some embodiments,the carrier liquid 152 is water. The combined gallium shot 150 andcarrier liquid 152 may then be heated, prior to or during agitation, toa temperature within a desired range. The temperature range in thisparticular example may be between about 40° C. to about 110° C.Preferably, the temperature will be 100° C. or greater. The heatedmaterial 154 may then be agitated to create the gallium emulsion 156.The agitation may be by sonication, vibration, stirring, manipulation offluid flow, and/or combinations thereof, or any other combination ofmechanical, electromagnetic, or acoustic means.

Referring still to FIG. 5, the group IA-based material added to thegallium emulsion 156 may be prepared by adding the group IA shots 160into a carrier liquid 162. The carrier liquid 162 may be a solventand/or include a surfactant. Optionally, the carrier liquid 162 may be anon-organic solvent. In some embodiments, the carrier liquid 162 iswater. The combined group IA shots 160 and carrier liquid 162 may thenbe heated to a temperature sufficient to liquefy the group IA shots 160.The temperature will preferably be 100° C. or greater. The heatedmaterial 164 may then be agitated to create the group IA-based emulsion166. The agitation may be sonication, stirring, manipulating of fluidflow, mechanical churning, and combinations thereof, or any othercombination of mechanical, electromagnetic, or acoustic means.

In one embodiment of the present invention, the gallium emulsion 156 andthe group IA emulsion 166, e.g. a Na emulsion, may be combined to formthe Ga₄Na dispersion 170 which may be dried to obtain dry Ga₄Naparticles. In other embodiments, it should be understood that dry Na orgroup IA element powder may be used in place of and/or in combinationwith the group IA emulsion 166. This may provide sufficient amounts ofsodium or group IA element to reach the desired stoichiometry to formthe desired group IIIA-IA based particles. Note that it might beadvantageous to make the Ga-emulsion at rt (or 40° C.), make theNa-emulsion >100° C., subsequently, cool-down one or both to have eitherone or both as solid particles during the alloying in case the alloyingis extremely exothermic (lot of heat generated, possibly causing anuncontrollable reaction). If the reaction is easily controlled, than inview of time, having both >100° C. would be the best (liquid-liquidchemistry typically faster than liquid-solid or solid-solid).

Referring now to FIGS. 6A and 6B, the emulsion technique may lead tosmaller particles than the gallium droplets 180 in the initialGa-emulsion 156. This may be the result of the slow addition of thesodium emulsion into the emulsion of gallium, or vice versa. The galliumwill diffuse into the (smaller) sodium droplets in emulsion 166 or Ga₄Nashells or cores will be found on or inside the gallium droplets 182 (seeFIG. 6B) which will then be broken up by sonication or other processeswhile exposing new un-reacted gallium in smaller droplets 184. Theportions broken off will likely be smaller particles and the continuedexposure of unreacted gallium will continue to create other smallparticles. Furthermore, it is possible that milling be used as theagitation source in this case since any coating from liquid gallium onmill part will come off when solidified. As seen in FIG. 6B, some of thesolidified particles 186 of Ga₄Na will settle to the bottom of thecontainer if unagitated. Of course, other techniques besides sonicationmay also be used to size reduce and/agitate the globules in theemulsion. Other such techniques include but are not limited to:mechanical agitation, high pressure homogenization using a device suchas an Emulsiflex™, available from Avestin, Inc., agitation via stirring,and emulsification via (horizontal) bead-milling, manipulation of fluidflow, stirring, combinations of the foregoing, or the like.

Note that forming solid In particles by quenching an emulsion of In ispossible as well, from a particle synthesis and size control point ofview. The process may involve making an In-emulsion and quenching theemulsion by adding a compound that acts as a seed to solidify it intosmaller particles than the In-nanoglobules.

Bandgap Grading Using IIIA-Based Materials

Referring now to FIGS. 7A-7C, a still further embodiment of the presentinvention will now be described. This embodiment describes a method ofselecting materials that have different reaction rates, due to e.g. ahigher thermodynamic stability compared to the other materials used, toachieve a desired distribution of material in the photovoltaic device.This may allow for fine tuning of the resulting photovoltaic device bycontrolling where materials are located in the device layers. By way ofnonlimiting example, the reaction rate of the group IIIA-IA based alloymay be sufficiently high such that it does not react until otherportions of the precursor material have begun to react. For Ga₄Na andGa₃₉Na₂₂, they melt at the selenization temperature and above the annealtemperature for other materials used for CIGS formation via RTP. Forexample, the melting point of Ga₄Na and Ga₃₉Na₂₂ is much higher than thetypical decomposition, melting, and reaction temperatures of most of theintermetallic alloys and chalcogenides present, formed, and consumedduring the absorber growth process(es). This advantageously keeps thesematerials at or near the locations where these materials were depositedand not letting them migrate as they tend to do when they react at thesame time as the other materials.

As seen in FIG. 7A, one embodiment of the present invention involvescoating a dispersion 200 of the solid group IIIA-IA based particles overthe precursor layer 202. In this example, the precursor layer 202 may bea layer of un-annealed material over a conductive layer 204 and asubstrate 206. Although not limited to the following, the particles inlayer 202 may be all spherical particles, all non-spherical particles,and/or mixtures of particles of various shapes. When heated as shown inFIG. 7B, the precursor layer 202 fuses into the annealed layer 208. FIG.7B continues to show that the coating of the dispersion 200 continues tohave particles of solid group IIIA-IA based material. The dispersion 200with the solid particles of group IIIA-IA based material has not reacheda sufficient temperature to react the particles therein significantly,or at least not to the same extent as the particles in 202. The layersof materials continues to be heated from the configuration of FIG. 7B tothat of 7C. FIG. 7C shows that continued heating at a higher temperatureor for a longer time results in the reacting of the coating of thedispersion 200 and formation of a layer 210 similar to the layer 208,except that the layer 210 has an increased group IIIA and group IAconcentration. The annealed layers 208 and 210 may be part of a two-stepor multi-step processing technique where further processing is requiredto turn the layers into an absorber layer. Thus for a CIGS absorberlayer, the layers 208 and 210 may require further selenization to becomea CIGS absorber layer. Due to the differing compositions of the layers208 and 210 (i.e. higher group IIIA content in layer 210), a desiredbandgap grading will be retained in the resulting absorber layer. Thestoichiometry will be such that there is a higher ratio of a group IIIAelement such as but not limited to gallium in the layer 210 than inlayer 208 to obtain the desired performance and/or bandgap. In oneembodiment, the desired stoichiometric ratio in layer 208 for band gapgrading may include but is not limited to a Ga/(In+Ga) at the back of208 (and 210) in the range of 0.3-0.7, close to the top surface of 210 aGa/(In+Ga) of 0.1-0.4, and at the top surface of 210 a Ga/(In+Ga) in therange of 0.15-0.45. Regarding Cu/(In+Ga); in the bulk <1.0.

Referring now to FIGS. 8A-8C, yet another embodiment of the presentinvention will now be described. This embodiment of the presentinvention involves coating a dispersion 220 of the solid group IIIA-IAbased particles over a precursor layer 222. In this example, theprecursor layer 222 may be a layer of unannealed material over aconductive layer 204 and a substrate 206, or may contain one or moreevaporated layers including selenium. The conductive layer 204 may beconductive electrically, thermally, or both. When heated as shown inFIG. 8B, the precursor layer 222 fuses into the annealed layer 228. Inthe example shown in FIG. 8B, the layer 228 may be a CIS layer. Thelayer 220 has not reached a sufficient temperature to reactsignificantly or at least not to the same extent as the particles in222. The embodiment of FIG. 8B is heated to a higher temperature or fora longer time, where it then reacts with the layer 228 to form layer 230in FIG. 8C. As seen in FIG. 8C, the layer 230 may be a CIGS absorberlayer. It may have a higher concentration of group IIIA elements nearthe top surface formerly occupied by layer 220. The difference betweenlayer 228 of FIG. 8B and layer 208 of FIG. 7B is the inclusion of agroup VIA element in layer 228. The layer 228 may be part of a one-stepprocessing technique where further processing is not required to turnthe resulting layer 230 into an absorber layer after annealing of layers220 and 228.

Referring now to FIGS. 9A and 9B, bandgap grading may also be applied ontop of a fully formed absorber layer 230. FIG. 9A shows that the presentinvention involves coating a dispersion 200 of the solid group IIIA-IAbased particles over a CIGS absorber layer 230. An extra layer 240 ofgroup VIA material such as but not limited to selenium may be used withthe dispersion 200, especially if the group IIIA-IA based material doesnot contain any or sufficient amounts of group VIA material in coatingof the dispersion 200. If the coating of the dispersion 200 is a solidgroup IIIA-IA-VIA based material, then this layer 240 may or may not benecessary depending on the content therein. FIG. 9B shows that uponsufficient heating, the resulting layer 250 may be formed as a CIGSlayer with a graded bandgap.

FIGS. 10A-10B shows yet another variation similar to that of FIGS.9A-9B. Instead of using a group IIIA-IA based material, layer 260 uses agroup IIIA-IA-VIA based material and thus a separate layer 240 of groupVIA material is not used. The resulting layer 250 will have the desiredbandgap grading and is similar to formed in FIG. 9B.

As seen from the foregoing, a variety of methods may be used to obtainbandgap grading using materials that will react after the sub-layershave begun to at least partially anneal. As nonlimiting examples, Ga—Na(any Na concentration if thin enough) or Ga—Se—Na may be used on top ofeither precursor CIG, CIGS, CI, annealed CIG, CI (from elements oralloys), or on top of a selenized layer which could even be copper richfor crystal growth or other purposes. In another embodiment, coating ofa Ga—Na or similar layer on top of a selenized layer is not verydifferent from a process point of view from solution deposition of agallium emulsion, except that thin coatings from Ga—Na are likely easierto coat than a gallium emulsion containing larger droplets and/orsensitive to coalescence. The dewetting risk is low since liquid is notformed until the temperature where the gallium is likely to incorporateinto the film, which is higher than the anneal temperature. In a stillfurther embodiment, coating this material on top of a precursor layer(prior to selenization or anneal) is likely to have an advantage in thatgallium is less likely to diffuse into the bulk because it will be inthe solid state at low temperatures. Then at the melting temperature,the temperature is high enough for good CIGS formation which is likelyto freeze the gallium at the top. Optionally, it should be understoodthat other embodiments may also mix the bandgap grading material in theprecursor layer, in addition to or in place of additional bandgapmaterial above the precursor layer.

Additionally, some embodiments may use Ga—Na particles that are notcompletely solid particles. At about 7.6 weight percent, the alloy canbe stable as Ga₄Na and be fully solid. Compounds containing loweramounts of sodium may contain portions that separate out from thecomposition into elemental gallium and Ga₄Na. At higher weightpercentages such as about 15.7% Na, the alloy may be stable as Ga₃₉Na₂₂.Some embodiments may use Na at weight percentages greater than about15.7%. These may have some separation but still provide the desired bandgap grading. Optionally, elements of other group IA-IIIA material mayalso be incorporated into the particle to prevent separation ofundesired elemental materials. These group IA-IIIA materials may bedeposited above, with, and/or below the precursor material.

It should be understood that a binary or multi-nary alloy(IB-IIIA-VIA-IA) in a broad range of compositions may be a combinationof two or more line-compounds and solid-solutions, where one or morecompounds might be present as a liquid at processing temperature. As anon-limiting example, Ga—Na at 300 C with a composition of 15 atomicpercent Na consists mainly of solid Ga₄Na (at thermodynamic equilibrium,see phase diagram) and some, almost pure, liquid gallium. Anothernonlimiting example, Ga—Na at 200 C with a composition of 28 atomicpercent sodium consists of a mixture of two different solids, beingGa₄Na and Ga₃₉Na₂₂. In other words, a material with a formula like e.g.Cu—In—Ga (with or without details on the actual stoichiometry of thebulk material) might consist of one, but more commonly, a mixture of twoor more different compounds of different compositions.

It should be understood that in some embodiments, part of the precursormaterial is allowed to liquefy, meaning starting with a composition ofGa—Na that will result in both nanodroplets of elemental-Ga and solidGa₄Na particles. The same holds for In—Na (although, liquefyingelemental-In occurs at 156° C.). In other words in some embodiments ofthe present invention, the precursor material containing the solidIIIA-alloy may contain liquid material next to the solid IIIA-alloyprior to, during, or after particle synthesis. In some embodiments ofthe present invention, the same holds for ink preparation, inkdeposition, and conversion to a compound layer.

Particle Shapes

It should be understood that any of solid particles as discussed hereinmay be used in spherical and/or non-spherical particle shapes. FIG. 1Ashows that the particles may all be non-spherical, planar flakeparticles. By way of example and not limitation, it should be understoodthat the solid Group IIIA-based particles may be particles of variousshapes used with any of the combinations shown below in Table II. Flakesmay be considered to be one type of non-spherical particles.

TABLE II Spherical Non-Spherical Flake Nanoglobules Spherical SphericalNon-spherical + Flake + Nanoglobules + Spherical Spherical SphericalNon- Spherical + Non-spherical Flake + Nanoglobules + Spherical Non-Non-spherical Non-spherical spherical Flake Spherical + Non-spherical +Flake Nanoglobules + Flake Flake Flake Nano- Spherical + Non-spherical +Flake + Nanoglobules globules Nano- Nanoglobules Nanoglobules globulesSpherical + Spherical + Spherical + Spherical + Spherical + Non- Non-Non-spherical Non-spherical + Non-spherical + spherical spherical FlakeNanoglobules Spherical + Spherical + Spherical + Spherical + Spherical +Flake Flake Flake + Flake Flake + Non-spherical Nanoglobules Spherical +Spherical + Spherical + Spherical + Spherical + Nano- Nano-Nanoglobules + Nanoglobules + Nanoglobules globules globulesNon-spherical Flake Flake + Flake + Flake + Flake + Flake + Non- Non-Nonspherical Nonspherical Nonspherical + spherical spherical +Nanoglobules Spherical Flake + Flake + Flake + Flake + Flake + Nano-Nano- Nanoglobules + Nanoglobules Nanoglobules globules globules +Non-spherical Spherical Non- Non- Non-spherical + Non-spherical +Non-spherical + spherical + spherical + Nanoglobules Nanoglobules +Nanoglobules Nano- Nano- Flake globules globules + Spherical

FIG. 11 shows one embodiment of the present invention where sphericalparticles 280 and oblong particles 282 (i.e. one type of non-sphericalparticles) are shown in combination. They may be deposited together orsequentially to form precursor layer 284.

FIG. 12 shows yet another embodiment of the present invention whereparticles of one time are used in combination with particles of anothertype. FIG. 12 shows that planar, flake particles 290 maybe used withspherical particles 292 to form the precursor layer 294.

Additional Sodium

Referring now to FIGS. 13A-13E, it should be understood that even withsolid group IIIA-based particles, more sodium may be desired in certainembodiments of the present invention to provide improved performance.This embodiment of the invention shows that layers of material may bedeposited above and/or below the precursor layer. Some layers may bedeposited after the precursor layer has been processed. Although notlimited to the following, these layers may provide one technique foradding additional sodium.

Referring now to FIG. 13A, the absorber layer may be formed on asubstrate 312, as shown in FIG. 13A. A surface of the substrate 312 maybe coated with a contact layer 314 to promote electrical contact betweenthe substrate 312 and the absorber layer that is to be formed on it. Byway of example, an aluminum substrate 312 may be coated with a contactlayer 314 of molybdenum. As discussed herein, forming or disposing amaterial or layer of material on the substrate 312 includes disposing orforming such material or layer on the contact layer 314, if one is used.Optionally, it should also be understood that a layer 315 may also beformed on top of contact layer 314 and/or directly on substrate 312. Aninterlayer may also be incorporated as previously described. This layermay be solution coated, evaporated, and/or deposited using vacuum basedtechniques. Although not limited to the following, the layer 315 mayhave a thickness less than that of the precursor layer 316. In onenonlimiting example, the layer may be between about 1 nm to about 100 nmin thickness. The layer 315 may be comprised of various materialsincluding but not limited to at least one of the following: a group IBelement, a group IIIA element, a group VIA element, a group IA element(new style: group 1), a binary and/or multinary alloy of any of thepreceding elements, a solid solution of any of the preceding elements,copper, indium, gallium, selenium, copper indium, copper gallium, indiumgallium, sodium, a sodium compound, sodium fluoride, sodium indiumsulfide, copper selenide, copper sulfide, indium selenide, indiumsulfide, gallium selenide, gallium sulfide, copper indium selenide,copper indium sulfide, copper gallium selenide, copper gallium sulfide,indium gallium selenide, indium gallium sulfide, copper indium galliumselenide, and/or copper indium gallium sulfide.

As shown in FIG. 13B, a precursor layer 316 is formed on the substrate.The precursor layer 316 contains one or more group IB elements and oneor more group IIIA elements. Preferably, the one or more group IBelements include copper. The one or more group IIIA elements may includeindium and/or gallium. The precursor layer may be formed using any ofthe techniques described above. In one embodiment, the precursor layercontains no oxygen other than those unavoidably present as impurities orincidentally present in components of the film other than the nano- ormicroflakes themselves. Although the precursor layer 316 is preferablyformed using non-vacuum methods, it should be understood that it mayoptionally be formed by other means, such as but not limited to,evaporation, sputtering, chemical vapor deposition, physical vapordeposition, atomic layer deposition (ALD), etc. By way of example, theprecursor layer 316 may be an oxygen-free compound containing copper,indium and gallium. In one embodiment, the non-vacuum system operates atpressures above about 3.2 kPa (24 Torr). Optionally, it should also beunderstood that a layer 317 may also be formed on top of precursor layer316. It should be understood that the stack may have both layers 315 and317, only one of the layers, or none of the layers. Although not limitedto the following, the layer 317 may have a thickness less than that ofthe precursor layer 316. In one nonlimiting example, the layer may bebetween about 1 to about 100 nm in thickness. The layer 317 may becomprised of various materials including but not limited to at least oneof the following: a group IB element, a group IIIA element, a group VIAelement, a group IA element (new style: group 1), a binary and/ormultinary alloy of any of the preceding elements, a solid solution ofany of the preceding elements, copper, indium, gallium, selenium, copperindium, copper gallium, indium gallium, sodium, a sodium compound,sodium fluoride, sodium indium sulfide, copper selenide, copper sulfide,indium selenide, indium sulfide, gallium selenide, gallium sulfide,copper indium selenide, copper indium sulfide, copper gallium selenide,copper gallium sulfide, indium gallium selenide, indium gallium sulfide,copper indium gallium selenide, and/or copper indium gallium sulfide.

Referring now to FIG. 13C, heat 320 is applied to anneal the firstprecursor layer 316 into a group IB-IIIA compound film 318. It should beunderstood that this may also include some amounts of IA material, butthe IA material is typically at a level that does not change the mainIB-IIIA material. In one nonlimiting example, the group IA material isless than about 3% at. of the composition in the precursor material andless than 0.1% of the final semiconductor material that contains groupIA material. Optionally, some may have less than about 2% at. of thecomposition in the precursor material. Optionally, some may have lessthan about 1% at. of the composition in the precursor material. In manyembodiments, most of the excess IA material is not in the final film butcan be found along the boundaries. Optionally, the IA material isincluded as a dopant. Optionally, some embodiments may have significantamounts of Na and thus create a group IA-IB-IIIA compound film which inthe final embodiment may be a IA-IB-IIIA-VIA. Optionally, the IAmaterial may be such that it helps crystal growth for the IB-IIIA-VIAmaterial, but the IA material is mainly gone in the final semiconductorfilm.

Referring still to the embodiment of FIG. 13C, the heat 320 may besupplied in a rapid thermal annealing process, e.g., as described above.As a nonlimiting example, the substrate 312 and precursor layer(s) 316may be heated from an ambient temperature to a plateau temperature rangeof between about 200° C. and about 600° C. The heat 320 may be suppliedin a rapid thermal annealing process, e.g., as described above. As anon-limiting example, the substrate 312 and precursor layer(s) 316 maybe heated from an ambient temperature to a plateau temperature range ofbetween about 200° C. and about 600° C. Processing comprises annealingwith a ramp-rate of about 1-5° C./sec, optionally over about 5° C./sec,to a temperature of about 200° C. and about 600° C. The temperature ismaintained in the plateau range for a period of time ranging betweenabout a fraction of a second to about 60 minutes, and subsequentlyreduced. Optionally in some embodiments, there are embodimentscontemplated wherein the ramp-down and ramp-up between the H2-anneal andselenization is avoided. In one such embodiment, there does not includea step where temperature is reduced to room temperature and/ortemperatures less than 100 C. In addition, some embodiments of thepresent invention may use heating of the as-coated CIG (IB-IIIA) and/oras-annealed CIG (IB-IIIA) without heating the substrate by using alaser. Optionally, processing further comprise selenizing this annealedlayer with a ramp-rate of about 1-5° C./sec, optionally over about 5°C./sec, to a temperature of about 225 to about 600° C. for a time periodof about 60 seconds to about 10 minutes in Se vapor, where the plateautemperature is not necessarily kept constant in time, to form thethin-film containing one or more chalcogenide compounds containing Cu,In, Ga, and Se. Optionally, processing comprises selenizing without theseparate annealing step in an atmosphere containing hydrogen gas, butmay be densified and selenized in one step with a ramp-rate of 1-5°C./sec, preferably over 5° C./sec, to a temperature of 225 to 600° C.for a time period of about 120 seconds to about 20 minutes in anatmosphere containing either H₂Se or a mixture of H₂ and Se vapor. Theheat turns the precursor layer into a film 322. Optionally, this may bea dense, metallic film as shown in FIG. 13D. The heating may removevoids and create a denser film than the precursor layer. In otherembodiments, where the precursor layer is already dense, there may belittle to no densification.

Optionally, as shown in FIG. 13D, a layer 326 containing an additionalchalcogen source, and/or an atmosphere containing a chalcogen source,may optionally be applied to layer 322. Heat 328 may optionally beapplied to layer 322 and the layer 326 and/or atmosphere containing thechalcogen source to heat them to a temperature sufficient to melt thechalcogen source and to react the chalcogen source with the group IBelement and group IIIA elements in the precursor layer 322. The heat 328may be applied in a rapid thermal annealing process, e.g., as describedabove. The reaction of the chalcogen source with the group IB and IIIAelements forms a compound film 330 of a group IB-IIIA-chalcogenidecompound as shown in FIG. 13E. Preferably, the groupIB-IIIA-chalcogenide compound is of the formCu_(z)In_(1-x-)Ga_(x)Se_(2(1-y))S_(2y), where 0≦x≦1, 0≦y≦1, and0.5≦y≦1.5. Although not limited to the following, the compound film 330may be thicker than the film 322 due to the reaction with group VIAelements.

Referring now to FIGS. 13A-13E, it should be understood that sodium mayalso be used with the precursor material to improve the qualities of theresulting film. This may be particularly useful in the situation wheresolid Group IIIA particles are formed without using a sodium basedmaterial and additional sodium is desired. In a first method, asdiscussed in regards to FIGS. 13A and 13B, one or more layers of asodium containing material may be formed above and/or below theprecursor layer 316. The formation may occur by solution coating and/orother techniques such as but not limited to sputtering, evaporation,chemical bath deposition (CBD, electroplating, sol-gel based coating,spray coating, chemical vapor deposition (CVD), physical vapordeposition (PVD), atomic layer deposition (ALD), and the like.

Optionally, in a second method, sodium may also be introduced into thestack by sodium doping the nanoflakes microflakes and/or particles inthe precursor layer 316. As a nonlimiting example, the nanoflakes and/orother particles in the precursor layer 316 may be a sodium containingmaterial such as, but not limited to, Cu—Na, In—Na, Ga—Na, Cu—In—Na,Cu—Ga—Na, In—Ga—Na, Na—Se, Cu—Se—Na, In—Se—Na, Ga—Se—Na, Cu—In—Se—Na,Cu—Ga—Se—Na, In—Ga—Se—Na, Cu—In—Ga—Se—Na, Na—S, Cu—In—Ga—Na, Cu—S—Na,In—S—Na, Ga—S—Na, Cu—In—S—Na, Cu—Ga—S—Na, In—Ga—S—Na, and/orCu—In—Ga—S—Na. In one embodiment of the present invention, the amount ofsodium in the nanoflakes or microflakes and/or other particles may beabout 1 atomic percent (at. %) or less. In another embodiment, theamount of sodium may be about 0.5 at. % or less. In yet anotherembodiment, the amount of sodium may be about 0.1 at. % or less. Itshould be understood that the doped particles and/or flakes may be madeby a variety of methods including milling feedstock material with thesodium containing material and/or elemental sodium.

Optionally, in a third method, sodium may be incorporated into the inkitself, regardless of the type of particle, nanoparticle, microflake,and/or nanoflakes dispersed in the ink. As a nonlimiting example, theink may include nanoflakes (Na doped or undoped) and a sodium compoundwith an organic counter-ion (such as but not limited to sodium acetate)and/or a sodium compound with an inorganic counter-ion (such as but notlimited to sodium sulfide). It should be understood that sodiumcompounds added into the ink (as a separate compound), might be presentas particles (e.g. nanoparticles), or dissolved and/or in (reverse)micelles. The sodium may be in “aggregate” form of the sodium compound(e.g. dispersed particles), and the “molecularly dissolved” form.Finally this added sodium may incorporate into the particles by themilling process or by any number of alloying processes described above.

None of the three aforementioned methods are mutually exclusive and maybe applied singly or in any single or multiple combination(s) to providethe desired amount of sodium to the stack containing the precursormaterial. Additionally, sodium and/or a sodium containing compound mayalso be added to the substrate (e.g. into the molybdenum target). Also,sodium-containing layers may be formed in between one or more precursorlayers if multiple precursor layers (using the same or differentmaterials) are used. It should also be understood that the source of thesodium is not limited to those materials previously listed. Some mayinclude other sodium compounds such as NaBF4, NaPF6, and/or sodiumtetraphenlborate. As a nonlimiting example, basically, any deprotonatedalcohol where the proton is replaced by sodium, any deprotonated organicand/or inorganic acid being, the sodium salt of the (deprotonated) acidcan be used, Na_(x)H_(y)Se_(z)S_(u)Te_(v)O_(w) where x, y, z, u, v, andw≧O, Na_(x)Cu_(y)In_(z)Ga_(u)O_(v) where x, y, z, u, and v≧0 sodiumhydroxide, sodium acetate, and the sodium salts of the following acids:butanoic acid, hexanoic acid, octanoic acid, decanoic acid, dodecanoicacid, tetradecanoic acid, hexadecanoic acid, 9-hexadecenoic acid,octadecanoic acid, 9-octadecenoic acid, 11-octadecenoic acid,9,12-octadecadienoic acid, 9,12,15-octadecatrienoic acid, and/or6,9,12-octadecatrienoic acid.

Optionally, as seen in FIG. 13E, it should also be understood thatsodium and/or a sodium compound may be added to the processedchalcogenide film after the precursor layer has been annealed orotherwise processed. This embodiment of the present invention thusmodifies the film after CIGS formation. With sodium, carrier trap levelsassociated with the grain boundaries are reduced, permitting improvedelectronic properties in the film. A variety of sodium containingmaterials such as those listed above may be deposited as layer 432 ontothe processed film and then annealed to treat the CIGS film.

Additionally, the sodium material may be combined with other elementsthat can provide a bandgap widening effect. Two elements which wouldachieve this include gallium and sulfur. The use of one or more of theseelements, in addition to sodium, may further improve the quality of theabsorber layer. The use of a sodium compound such as but not limited toNa₂S, NaInS₂, or the like provides both Na and S to the film and couldbe driven in with an anneal such as but not limited to an RTA step toprovide a layer with a bandgap different from the bandgap of theunmodified CIGS layer or film.

Roll-to-Roll Manufacturing

Referring now to FIG. 14, a roll-to-roll manufacturing process accordingto the present invention will now be described. Embodiments of theinvention using the solid group IIIA-based materials are well suited foruse with roll-to-roll manufacturing. Specifically, in a roll-to-rollmanufacturing system 400 a flexible substrate 401, e.g., aluminum foiltravels from a supply roll 402 to a take-up roll 404. The width of therow may vary depending on the application. Some embodiments have aroll(s) with a width greater than 0.5 m, greater than 1.0 m, greaterthan 2.0 m, and/or greater than 3.0 m or more. These substrates may behigh aspect ratio with lengths substantially greater than the width suchas but not limited to aspect ratios of 50:1, 100:1, or more. In betweenthe supply and take-up rolls, the substrate 401 passes a number ofapplicators 406A, 406B, 406C, e.g. microgravure rollers and heater units408A, 408B, 408C. Each applicator deposits a different layer orsub-layer of a precursor layer, e.g., as described above. The heaterunits are used to anneal the different layers and/or sub-layers to formdense films. In the example depicted in FIG. 14, applicators 406A and406B may apply different sub-layers of a precursor layer. Heater units408A and 408B may anneal each sub-layer before the next sub-layer isdeposited. Alternatively, both sub-layers may be annealed at the sametime. Applicator 406C may optionally apply an extra layer of materialcontaining chalcogen or alloy or elemental particles as described above.Heater unit 408C heats the optional layer and precursor layer asdescribed above. Note that it is also possible to deposit the precursorlayer (or sub-layers) then deposit any additional layer and then heatall three layers together to form the IB-IIIA-chalcogenide compound filmused for the photovoltaic absorber layer. The roll-to-roll system may bea continuous roll-to-roll and/or segmented roll-to-roll, and/or batchmode processing.

Photovoltaic Device

Referring now to FIG. 15A, the films fabricated as described above usingsolid group IIIA-based materials may serve as an absorber layer in aphotovoltaic device, module, or solar panel. An example of such aphotovoltaic device 450 is shown in FIG. 14. The device 450 includes abase substrate 452, an optional adhesion layer 453, a base or backelectrode 454, a p-type absorber layer 456 incorporating a film of thetype described above, a n-type semiconductor thin film 458 and atransparent electrode 460. By way of example, the base substrate 452 maybe made of a metal foil, a polymer such as polyimides (PI), polyamides,polyetheretherketone (PEEK), Polyethersulfone (PES), polyetherimide(PEI), polyethylene naphtalate (PEN), Polyester (PET), related polymers,a metallized plastic, and/or combination of the above and/or similarmaterials. By way of nonlimiting example, related polymers include thosewith similar structural and/or functional properties and/or materialattributes. The base electrode 454 is made of an electrically conductivematerial. By way of example, the base electrode 454 may be of a metallayer whose thickness may be selected from the range of about 0.1 micronto about 25 microns. An optional intermediate layer 453 may beincorporated between the electrode 454 and the substrate 452. Thetransparent electrode 460 may include a transparent conductive layer 459and a layer of metal (e.g., Al, Ag, Cu, or Ni) fingers 461 to reducesheet resistance. Optionally, the layer 453 may be a diffusion barrierlayer to prevent diffusion of material between the substrate 452 and theelectrode 454. The diffusion barrier layer 453 may be a conductive layeror it may be an electrically nonconductive layer. As nonlimitingexamples, the layer 453 may be composed of any of a variety ofmaterials, including but not limited to chromium, vanadium, tungsten,and glass, or compounds such as nitrides (including tantalum nitride,tungsten nitride, titanium nitride, silicon nitride, zirconium nitride,and/or hafnium nitride), oxides, carbides, and/or any single or multiplecombination of the foregoing. As nonlimiting examples, the layer 453 maybe composed of any of a variety of materials, including but not limitedto chromium, vanadium, tungsten, and glass, or compounds such asnitrides (including tantalum nitride, tungsten nitride, titaniumnitride, silicon nitride, zirconium nitride, and/or hafnium nitride),oxides, carbides, and/or any single or multiple combination of theforegoing. Although not limited to the following, the thickness of thislayer can range from 10 nm to 500 nm. In some embodiments, the layer maybe from 100 nm to 300 nm. Optionally, the thickness may be in the rangeof about 150 nm to about 250 nm. Optionally, the thickness may be about200 nm. In some embodiments, two barrier layers may be used, one on eachside of the substrate 452. Optionally, an interfacial layer may belocated above the electrode 454 and be comprised of a material such asincluding but not limited to chromium, vanadium, tungsten, and glass, orcompounds such as nitrides (including tantalum nitride, tungstennitride, titanium nitride, silicon nitride, zirconium nitride, and/orhafnium nitride), oxides, carbides, and/or any single or multiplecombination of the foregoing.

The n-type semiconductor thin film 458 serves as a junction partnerbetween the compound film and the transparent conducting layer 459. Byway of example, the n-type semiconductor thin film 458 (sometimesreferred to as a junction partner layer) may include inorganic materialssuch as cadmium sulfide (CdS), zinc sulfide (ZnS), zinc hydroxide, zincselenide (ZnSe), n-type organic materials, or some combination of two ormore of these or similar materials, or organic materials such as n-typepolymers and/or small molecules. Layers of these materials may bedeposited, e.g., by chemical bath deposition (CBD) and/or chemicalsurface deposition (and/or related methods), to a thickness ranging fromabout 2 nm to about 1000 nm, more preferably from about 5 nm to about500 nm, and most preferably from about 10 nm to about 300 nm. This mayalso configured for use in a continuous roll-to-roll and/or segmentedroll-to-roll and/or a batch mode system.

The transparent conductive layer 459 may be inorganic, e.g., atransparent conductive oxide (TCO) such as but not limited to indium tinoxide (ITO), fluorinated indium tin oxide, zinc oxide (ZnO) or zincoxide (ZnO_(x)) doped with aluminum, or a related material, which can bedeposited using any of a variety of means including but not limited tosputtering, evaporation, chemical bath deposition (CBD, electroplating,sol-gel based coating, spray coating, chemical vapor deposition (CVD),physical vapor deposition (PVD), atomic layer deposition (ALD), and thelike. Alternatively, the transparent conductive layer may include atransparent conductive polymeric layer, e.g. a transparent layer ofdoped PEDOT (Poly-3,4-Ethylenedioxythiophene), carbon nanotubes orrelated structures, or other transparent organic materials, eithersingly or in combination, which can be deposited using spin, dip, orspray coating, and the like or using any of various vapor depositiontechniques. Optionally, it should be understood that a non-conductivelayer such as intrinsic ZnO (i-ZnO) may be used between CdS and Al-dopedZnO. Optionally, an insulating layer may be included between the layer458 and transparent conductive layer 459. Combinations of inorganic andorganic materials can also be used to form a hybrid transparentconductive layer. Thus, the layer 459 may optionally be an organic(polymeric or a mixed polymeric-molecular) or a hybrid(organic-inorganic) material. Examples of such a transparent conductivelayer are described e.g., in commonly-assigned US Patent ApplicationPublication Number 20040187317, which is incorporated herein byreference.

Those of skill in the art will be able to devise variations on the aboveembodiments that are within the scope of these teachings. For example,it is noted that in embodiments of the present invention, portions ofthe IB-IIIA precursor layers (or certain sub-layers of the precursorlayers or other layers in the stack) may be deposited using techniquesother than microflake-based inks. For example precursor layers orconstituent sub-layers may be deposited using any of a variety ofalternative deposition techniques including but not limited tosolution-deposition of spherical nanopowder-based inks, vapor depositiontechniques such as ALD, evaporation, sputtering, CVD, PVD,electroplating and the like.

Referring now to FIG. 15B, it should also be understood that a pluralityof devices 450 may be incorporated into a module 500 to form a solarmodule that includes various packaging, durability, and environmentalprotection features to enable the devices 450 to be installed in anoutdoor environment. In one embodiment, the module 500 may include aframe 502 that supports a substrate 504 on which the devices 450 may bemounted. This module 500 simplifies the installation process by allowinga plurality of devices 450 to be installed at one time. Alternatively,flexible form factors may also be employed. It should also be understoodthat an encapsulating device and/or layers may be used to protect fromenvironmental influences. As a nonlimiting example, the encapsulatingdevice and/or layers may block the ingress of moisture and/or oxygenand/or acidic rain into the device, especially over extendedenvironmental exposure.

Referring now to FIG. 16A, it should also be understood that theembodiments of the present invention may also be used on a rigidsubstrate 600. By way of nonlimiting example, the rigid substrate 600may be glass, soda-lime glass, steel, stainless steel, aluminum,polymer, ceramic, coated polymer, or other rigid material suitable foruse as a solar cell or solar module substrate. A high speedpick-and-place robot 602 may be used to move rigid substrates 600 onto aprocessing area from a stack or other storage area. In FIG. 17A, thesubstrates 600 are placed on a conveyor belt which then moves themthrough the various processing chambers. Optionally, the substrates 600may have already undergone some processing by the time and may alreadyinclude a precursor layer on the substrate 600. Other embodiments of theinvention may form the precursor layer as the substrate 600 passesthrough the chamber 606.

FIG. 16B shows another embodiment of the present system where apick-and-place robot 610 is used to position a plurality of rigidsubstrates on a carrier device 612 which may then be moved to aprocessing area as indicated by arrow 614. This allows for multiplesubstrates 600 to be loaded before they are all moved together toundergo processing. Source 662 may provide a source of processing gas toprovide a suitable atmosphere to create the desired semiconductor film.In one embodiment, chalcogen vapor may be provided by using a partiallyor fully enclosed chamber with a chalcogen source 662 therein or coupledto the chamber.

Inter-Metallic Material

Referring now to FIG. 17, any of the foregoing solid particles,including solid group IIIA-based particles may be used with thefollowing inter-metallic materials. By way of example and notlimitation, the present invention also includes the possibility of usingsolid particles, emulsions of liquid materials, inter-metallicmaterials, and/or any single or multiple combinations of the foregoing.

In one embodiment, the particles used to form a precursor layer 1500 mayinclude particles that are inter-metallic particles 1502. In oneembodiment, an inter-metallic material is a material containing at leasttwo elements, wherein the amount of one element in the inter-metallicmaterial is less than about 50 molar percent of the total molar amountof the inter-metallic material and/or the total molar amount of that oneelement in a precursor material. The amount of the second element isvariable and may range from less than about 50 molar percent to about 50or more molar percent of the inter-metallic material and/or the totalmolar amount of that one element in a precursor material. Alternatively,inter-metallic phase materials may be comprised of two or more metalswhere the materials are admixed in a ratio between the upper bound ofthe terminal solid solution and an alloy comprised of about 50% of oneof the elements in the inter-metallic material. The particledistribution shown in the enlarged view of FIG. 17 is purely exemplaryand is nonlimiting. It should be understood that some embodiments mayhave particles that all contain inter-metallic materials, mixture ofmetallic and inter-metallic materials, metallic particles andinter-metallic particles, or combinations thereof.

It should be understood that inter-metallic phase materials arecompounds and/or intermediate solid solutions containing two or moremetals, which have characteristic properties and crystal structuresdifferent from those of either the pure metals or the terminal solidsolutions. Inter-metallic phase materials arise from the diffusion ofone material into another via crystal lattice vacancies made availableby defects, contamination, impurities, grain boundaries, and mechanicalstress. Upon two or more metals diffusing into one another, intermediatemetallic species are created that are combinations of the two materials.Sub-types of inter-metallic compounds include both electron andinterstitial compounds.

Electron compounds arise if two or more mixed metals are of differentcrystal structure, valency, or electropositivity relative to oneanother; examples include but are not limited to copper selenide,gallium selenide, indium selenide, copper telluride, gallium telluride,indium telluride, and similar and/or related materials and/or blends ormixtures of these materials.

Interstitial compounds arise from the admixture of metals or metals andnon-metallic elements, with atomic sizes that are similar enough toallow the formation of interstitial crystal structures, where the atomsof one material fit into the spaces between the atoms of anothermaterial. For inter-metallic materials where each material is of asingle crystal phase, two materials typically exhibit two diffractionpeaks, each representative of each individual material, superimposedonto the same spectra. Thus inter-metallic compounds typically containthe crystal structures of both materials contained within the samevolume. Examples include but are not limited to Cu—Ga, Cu—In, andsimilar and/or related materials and/or blends or mixtures of thesematerials, where the compositional ratio of each element to the otherplaces that material in a region of its phase diagram other than that ofthe terminal solid solution.

Inter-metallic materials are useful in the formation of precursormaterials for CIGS photovoltaic devices in that metals interspersed in ahighly homogenous and uniform manner amongst one another, and where eachmaterial is present in a substantially similar amount relative to theother, thus allowing for rapid reaction kinetics leading to high qualityabsorber films that are substantially uniform in all three dimensionsand at the nano-, micro, and meso-scales.

In the absence of the addition of indium nanoparticles, which aredifficult to synthesize and handle, terminal solid solutions do notreadily allow a sufficiently large range of precursor materials to beincorporated into a precursor film in the correct ratio (e.g.Cu/(In+Ga)=0.85) to provide for the formation of a highly lightabsorbing, photoactive absorber layer. Furthermore, terminal solidsolutions may have mechanical properties that differ from those ofinter-metallic materials and/or intermediate solid solutions (solidsolutions between a terminal solid solution and/or element). As anonlimiting example, some terminal solid solutions are not brittleenough to be milled for size reduction. Other embodiments may be toohard to be milled. The use of inter-metallic materials and/orintermediate solid solutions can address some of these drawbacks.

The advantages of particles 1502 having an inter-metallic phase aremulti-fold. As a nonlimiting example, a precursor material suitable foruse in a thin film solar cell may contain group IB and group IIIAelements such as copper and indium, respectively. If an inter-metallicphase of Cu—In is used such as Cu₁In₂, then Indium is part of an In-richCu material and not added as pure indium. Adding pure indium as ametallic particle is challenging due to the difficulty in achieving Inparticle synthesis with high yield, small and narrow nanoparticle sizedistribution, and requiring particle size discrimination, which addsfurther cost. Using inter-metallic In-rich Cu particles avoids pureelemental In as a precursor material. Additionally, because theinter-metallic material is Cu poor, this also advantageously allows Cuto be added separately to achieve precisely the amount of Cu desired inthe precursor material. The Cu is not tied to the ratio fixed in alloysor solid solutions that can be created by Cu and In. The inter-metallicmaterial and the amount of Cu can be fine tuned as desired to reach adesired stoichiometric ratio. Ball milling of these particles results inno need for particle size discrimination, which decreases cost andimproves the throughput of the material production process.

In some specific embodiments of the present invention, having aninter-metallic material provides a broader range of flexibility. Sinceeconomically manufacturing elemental indium particles is difficult, itwould be advantageous to have an indium-source that is more economicallyinteresting. Additionally, it would be advantageous if this indiumsource still allows varying both the Cu/(In+Ga) and Ga/(In+Ga) in thelayer independently of each other. As one nonlimiting example, adistinction can be made between Cu₁₁In₉ and Cu₁In₂ with aninter-metallic phase. This particularly true if only one layer ofprecursor material is used. If, for this particular example, if indiumis only provided by Cu₁₁In₉, there is more restriction whatstoichiometric ratio can be created in a final group IB-IIIA-VIAcompound. With Cu₁In₂ as the only indium source, however, there is muchgreater range of ratio can be created in a final group IB-IIIA-VIAcompound. Cu₁In₂ allows you to vary both the Cu/(In+Ga) and Ga/(In+Ga)independently in a broad range, whereas Cu11In9 does not. For instance,Cu11In9 does only allow for Ga/(In+Ga)=0.25 with Cu/(In+Ga)>0.92. Yetanother example, Cu11In9 does only allow for Ga/(In+Ga)=0.20 withCu/(In+Ga)>0.98. Yet another example, Cu11In9 does only allow forGa/(In+Ga)=0.15 with Cu/(In+Ga)>1.04. Thus for an intermetallicmaterial, particularly when the intermetallic material is a sole sourceof one of the elements in the final compound, the final compound may becreated with stoichiometric ratios that more broadly explore the boundsof Cu/(In+Ga) with a compositional range of about 0.7 to about 1.0, andGa/(In+Ga) with a compositional range of about 0.05 to about 0.3 Inother embodiments, Cu/(In+Ga) compositional range may be about 0.01 toabout 1.0. In other embodiments, the Cu/(In+Ga) compositional range maybe about 0.01 to about 1.1. In other embodiments, the Cu/(In+Ga)compositional range may be about 0.01 to about 1.5. This typicallyresults in additional Cu_(x)Se_(y) which we might be able to removeafterwards if it is at the top surface. It should be understood thatthese ratios may apply to any of the above embodiments described herein.

Furthermore, it should be understood that during processing, anintermetallic material may create more liquid than other compounds. As anonlimiting example, Cu₁In₂ will form more liquid when heated duringprocessing than Cu11In9. More liquid promotes more atomic intermixingsince it easier for material to move and mix while in a liquid stage.

Additionally, there are specific advantages for particular types ofinter-metallic particles such as, but not limited to, Cu₁In₂. Cu₁In₂ isa material that is metastable. The material is more prone todecomposition, which advantageously for the present invention, willincrease the rate of reaction (kinetically). Further, the material isless prone to oxidation (e.g. compared to pure In) and this furthersimplifies processing. This material may also be single-phase, whichwould make it more uniform as a precursor material, resulting in betteryield.

As seen in FIGS. 18 and 19, after the layer 1500 is deposited over thesubstrate 1506, it may then be heated in a suitable atmosphere to reactthe layer 1500 in FIG. 18 and form film 1510 shown in FIG. 19. It shouldbe understood that the layer 1500 may be used in conjunction with layers915 and 917 as described above with regards to FIG. 13A-13B. The layer915 may be comprised of various materials including but not limited atleast one of the following: a group IB element, a group IIIA element, agroup VIA element, a group IA element (new style: group 1), a binaryand/or multinary alloy of any of the preceding elements, a solidsolution of any of the preceding elements. It should be understood thatsodium or a sodium-based material such as but not limited to sodium, asodium compound, sodium fluoride, and/or sodium indium sulfide, may alsobe used in layer 915 with the precursor material to improve thequalities of the resulting film. FIG. 19 shows that a layer 932 may alsobe used as described with regards to FIG. 13F. Any of the methodsuggested previously with regards to sodium content may also be adaptedfor use with the embodiments shown in FIGS. 17-19.

It should be understood that other embodiments of the present inventionalso disclose material comprised of at least two elements wherein theamount of at least one element in the material is less than about 50molar percent of the total molar amount of that element in the precursormaterial. This includes embodiments where the amount of group IB elementis less than the amount of group IIIA element in inter-metallicmaterial. As a nonlimiting example, this may include other group IBpoor, group IB-IIIA materials such as Cu-poor Cu_(x)In_(y) particles(where x<y). The amount of group IIIA material may be in any range asdesired (more than about 50 molar percent of the element in theprecursor material or less than 50 molar percent). In anothernonlimiting example, Cu₁Ga₂ may be used with elemental Cu and elementalIn. Although this material is not an inter-metallic material, thismaterial is a intermediate solid solution and is different from aterminal solid solution. All solid particles are created based on aCu₁Ga₂ precursor. In this embodiment, no emulsions are used.

In still other embodiments of the present invention, other viableprecursor materials may be formed using a group IB rich, group IB-IIIAmaterial. As a nonlimiting example, a variety of intermediatesolid-solutions may be used. Cu—Ga (38 at % Ga) may be used in precursorlayer 1500 with elemental indium and elemental copper. In yet anotherembodiment, Cu—Ga (30 at % Ga) may be used in precursor layer 1500 withelemental copper and elemental indium. Both of these embodimentsdescribe Cu-rich materials with the Group IIIA element being less thanabout 50 molar percent of that element in the precursor material. Instill further embodiments, Cu—Ga (multiphasic, 25 at % Ga) may be usedwith elemental copper and indium to form the desired precursor layer. Itshould be understood that nanoparticles of these materials may becreated by mechanical milling or other size reduction methods. In otherembodiments, these particles may be made by electroexplosive wire (EEW)processing, evaporation condensation (EC), pulsed plasma processing, orother methods. Although not limited to the following, the particlessizes may be in the range of about 10 nm to about 1 micron. They may beof any shape as described herein.

Referring now to FIG. 20, in a still further embodiment of the presentinvention, two or more layers of materials may be coated, printed, orotherwise formed to provide a precursor layer with the desiredstoichiometric ratio. As a nonlimiting example, layer 1530 may contain aprecursor material having Cu₁₁In₉ and a Ga source such as elemental Gaand/or Ga_(x)Se_(y). A copper rich precursor layer 1532 containingCu₇₈In₂₈ (solid-solution) and elemental indium or In_(x)Se_(y) may beprinted over layer 1530. In such an embodiment, the resulting overallratios may have Cu/(In+Ga)=0.85 and Ga/(In+Ga) 0.19. In one embodimentof the resulting film, the film may have a stoichiometric ratio ofCu/(In+Ga) with a compositional range of about 0.7 to about 1.0 andGa/(In+Ga) with a compositional range of about 0.05 to about 0.3.

Referring now to FIG. 21, it should be understood that in someembodiments of the present invention, the inter-metallic material isused as a feedstock or starting material from which particles and/ornanoparticles may be formed. As a nonlimiting example, FIG. 21 shows oneinter-metallic feedstock particle 1550 being processed to form otherparticles. Any method used for size reduction and/or shape change may besuitable including but not limited to milling, EEW, EC, pulsed plasmaprocessing, or combinations thereof. Particles 552, 554, 556, and 558may be formed. These particles may be of varying shapes and some maycontain only the inter-metallic phase while others may contain thatphase and other material phases.

Referring now to FIGS. 22A and 22B, flakes 1600 (microflakes and/ornanoflakes) provide certain advantages over other non-spherical shapessuch as but not limited to platelets. The flakes 1600 provide for highlyefficient stacking (due to uniform thickness in Z-axis) and high surfacearea (in X and Y axes). This leads to faster reactions, better kinetics,and more uniform products/films/compounds (with fewer sidepropagations). Platelet 1602 as seen in FIGS. 23A and 23B fail to haveall of the above advantages.

Referring now to FIG. 24, shows that the foregoing discussion onintermetallics also applies to spherical particles. These sphericalintermetallic particles may be used with other spherical particles,non-spherical particles, particles of various shapes, and/or any singleor multiple combination of the foregoing.

In one embodiment, the particles used to form a precursor layer 1700 mayinclude particles that are inter-metallic particles 1702. In oneembodiment, an inter-metallic material is a material containing at leasttwo elements, wherein the amount of one element in the inter-metallicmaterial is less than about 50 molar percent of the total molar amountof the inter-metallic material and/or the total molar amount of that oneelement in a precursor material. The amount of the second element isvariable and may range from less than about 50 molar percent to about 50or more molar percent of the inter-metallic material and/or the totalmolar amount of that one element in a precursor material. Alternatively,inter-metallic phase materials may be comprised of two or more metalswhere the materials are admixed in a ratio between the upper bound ofthe terminal solid solution and an alloy comprised of about 50% of oneof the elements in the inter-metallic material. The particledistribution shown in the enlarged view of FIG. 24 is purely exemplaryand is nonlimiting. It should be understood that some embodiments mayhave particles that all contain inter-metallic materials, mixture ofmetallic and inter-metallic materials, metallic particles andinter-metallic particles, or combinations thereof.

It should be understood that inter-metallic phase materials arecompounds and/or intermediate solid solutions containing two or moremetals, which have characteristic properties and crystal structuresdifferent from those of either the pure metals or the terminal solidsolutions. Inter-metallic phase materials arise from the diffusion ofone material into another via crystal lattice vacancies made availableby defects, contamination, impurities, grain boundaries, and mechanicalstress. Upon two or more metals diffusing into one another, intermediatemetallic species are created that are combinations of the two materials.Sub-types of inter-metallic compounds include both electron andinterstitial compounds.

Electron compounds arise if two or more mixed metals are of differentcrystal structure, valency, or electropositivity relative to oneanother; examples include but are not limited to copper selenide,gallium selenide, indium selenide, copper telluride, gallium telluride,indium telluride, and similar and/or related materials and/or blends ormixtures of these materials.

Interstitial compounds arise from the admixture of metals or metals andnon-metallic elements, with atomic sizes that are similar enough toallow the formation of interstitial crystal structures, where the atomsof one material fit into the spaces between the atoms of anothermaterial. For inter-metallic materials where each material is of asingle crystal phase, two materials typically exhibit two diffractionpeaks, each representative of each individual material, superimposedonto the same spectra. Thus inter-metallic compounds typically containthe crystal structures of both materials contained within the samevolume. Examples include but are not limited to Cu—Ga, Cu—In, andsimilar and/or related materials and/or blends or mixtures of thesematerials, where the compositional ratio of each element to the otherplaces that material in a region of its phase diagram other than that ofthe terminal solid solution.

Inter-metallic materials are useful in the formation of precursormaterials for CIGS photovoltaic devices in that metals interspersed in ahighly homogenous and uniform manner amongst one another, and where eachmaterial is present in a substantially similar amount relative to theother, thus allowing for rapid reaction kinetics leading to high qualityabsorber films that are substantially uniform in all three dimensionsand at the nano-, micro, and meso-scales.

In the absence of the addition of indium nanoparticles, which aredifficult to synthesize and handle, terminal solid solutions do notreadily allow a sufficiently large range of precursor materials to beincorporated into a precursor film in the correct ratio (e.g.Cu/(In+Ga)=0.85) to provide for the formation of a highly lightabsorbing, photoactive absorber layer. Furthermore, terminal solidsolutions may have mechanical properties that differ from those ofinter-metallic materials and/or intermediate solid solutions (solidsolutions between a terminal solid solution and/or element). As anonlimiting example, some terminal solid solutions are not brittleenough to be milled for size reduction. Other embodiments may be toohard to be milled. The use of inter-metallic materials and/orintermediate solid solutions can address some of these drawbacks.

Referring now to FIG. 25, it should be understood that in someembodiments of the present invention, the inter-metallic material isused as a feedstock or starting material from which particles and/ornanoparticles may be formed. As a nonlimiting example, FIG. 25 shows oneinter-metallic feedstock particle 1750 being processed to form otherparticles. Any method used for size reduction and/or shape change may besuitable including but not limited to milling, EEW, EC, pulsed plasmaprocessing, or combinations thereof. Particles 1752, 1754, 1756, and1758 may be formed. These particles may be of varying shapes and somemay contain only the inter-metallic phase while others may contain thatphase and other material phases.

Again, any of the solid particles, including solid group IIIA-basedparticles may be used with the foregoing inter-metallic materials. Byway of example and not limitation, the present invention also includesthe possibility of using solid particles, emulsions of liquid materials,intermetallic materials, and/or any single or multiple combinations ofthe foregoing.

Chalcogenides

It should be understood that a variety of chalcogen and/or chalcogenideparticles may also be combined with non-chalcogenide particles to arriveat the desired excess supply of chalcogen in the precursor layer. Thefollowing table (Table IV) provides a non-limiting matrix of some of thepossible combinations between chalcogenide particles listed in the rowsand the non-chalcogenide particles listed in the columns. It should alsobe understood that two more materials from the columns may be combined.As a nonlimiting example, Cu—Ga+In+Se may also be combined even thoughthe are from different columns. Another possibility involves,Cu—Ga+In—Ga+Se (or some other chalcogen source).

TABLE IV Cu In Ga Cu—In Se Se + Cu Se + In Se + Ga Se + Cu—In Cu—SeCu—Se + Cu Cu—Se + In Cu—Se + Ga Cu—Se + Cu—In In—Se In—Se + Cu In—Se +In In—Se + Ga In—Se + Cu—In Ga—Se Ga—Se + Cu Ga—Se + In Ga—Se + GaGa—Se + Cu—In Cu—In—Se Cu—In—Se + Cu Cu—In—Se + In Cu—In—Se + GaCu—In—Se + Cu—In Cu—Ga—Se Cu—Ga—Se + Cu Cu—Ga—Se + In Cu—Ga—Se + GaCu—Ga—Se + Cu—In In—Ga—Se In—Ga—Se + Cu In—Ga—Se + In In—Ga—Se + GaIn—Ga—Se + Cu—In Cu—In—Ga—Se Cu—In—Ga—Se + Cu Cu—In—Ga—Se + InCu—In—Ga—Se + Ga Cu—In—Ga—Se + Cu—In Cu—Ga In—Ga Cu—In—Ga Se Se + Cu—GaSe + In—Ga Se + Cu—In—Ga Cu—Se Cu—Se + Cu—Ga Cu—Se + In—Ga Cu—Se +Cu—In—Ga In—Se In—Se + Cu—Ga In—Se + In—Ga In—Se + Cu—In—Ga Ga—SeGa—Se + Cu—Ga Ga—Se + In—Ga Ga—Se + Cu—In—Ga Cu—In—Se Cu—In—Se + Cu—GaCu—In—Se + In—Ga Cu—In—Se + Cu—In—Ga Cu—Ga—Se Cu—Ga—Se + Cu—GaCu—Ga—Se + In—Ga Cu—Ga—Se + Cu—In—Ga In—Ga—Se In—Ga—Se + Cu—GaIn—Ga—Se + In—Ga In—Ga—Se + Cu—In—Ga Cu—In—Ga—Se Cu—In—Ga—Se + Cu—GaCu—In—Ga—Se + In—Ga Cu—In—Ga—Se + Cu—In—Ga

In yet another embodiment, the present invention may combine a varietyof chalcogenide particles with other chalcogenide particles. Thefollowing table (Table V) provides a non-limiting matrix of some of thepossible combinations between chalcogenide particles listed for the rowsand chalcogenide particles listed for the columns.

TABLE V Cu—Se In—Se Ga—Se Cu—In—Se Se Se + Cu—Se Se + In—Se Se + Ga—SeSe + Cu—In—Se Cu—Se Cu—Se Cu—Se + In—Se Cu—Se + Ga—Se Cu—Se + Cu—In—SeIn—Se In—Se + Cu—Se In—Se In—Se + Ga—Se In—Se + Cu—In—Se Ga—Se Ga—Se +Cu—Se Ga—Se + In—Se Ga—Se Ga—Se + Cu—In—Se Cu—In—Se Cu—In—Se + Cu—SeCu—In—Se + In—Se Cu—In—Se + Ga—Se Cu—In—Se Cu—Ga—Se Cu—Ga—Se + Cu—SeCu—Ga—Se + In—Se Cu—Ga—Se + Ga—Se Cu—Ga—Se + Cu—In—Se In—Ga—SeIn—Ga—Se + Cu—Se In—Ga—Se + In—Se In—Ga—Se + Ga—Se In—Ga—Se + Cu—In—SeCu—In—Ga—Se Cu—In—Ga—Se + Cu—Se Cu—In—Ga—Se + In—Se Cu—In—Ga—Se + Ga—SeCu—In—Ga—Se + Cu—In—Se Cu—Ga—Se In—Ga—Se Cu—In—Ga—Se Se Se + Cu—Ga—SeSe + In—Ga—Se Se + Cu—In—Ga—Se Cu—Se Cu—Se + Cu—Ga—Se Cu—Se + In—Ga—SeCu—Se + Cu—In—Ga—Se In—Se In—Se + Cu—Ga—Se In—Se + In—Ga—Se In—Se +Cu—In—Ga—Se Ga—Se Ga—Se + Cu—Ga—Se Ga—Se + In—Ga—Se Ga—Se + Cu—In—Ga—SeCu—In—Se Cu—In—Se + Cu—Ga—Se Cu—In—Se + In—Ga—Se Cu—In—Se + Cu—In—Ga—SeCu—Ga—Se Cu—Ga—Se Cu—Ga—Se + In—Ga—Se Cu—Ga—Se + Cu—In—Ga—Se In—Ga—SeIn—Ga—Se + Cu—Ga—Se In—Ga—Se In—Ga—Se + Cu—In—Ga—Se Cu—In—Ga—SeCu—In—Ga—Se + Cu—Ga—Se Cu—In—Ga—Se + In—Ga—Se Cu—In—Ga—Se

Additionally, it should be understood that any number of combinations offlake and non-flake particles may be used according to the presentinvention in the various layers. As a nonlimiting example, thecombinations may include but are not limited to:

TABLE VI Combination 1  1) chalcogenide (flake) + non-chalcogenide(flake) Combination 2  2) chalcogenide (flake) + non-chalcogenide(non-flake) Combination 3  3) chalcogenide (non-flake) +non-chalcogenide (flake) Combination 4  4) chalcogenide (non-flake) +non-chalcogenide (non-flake) Combination 5  5) chalcogenide (flake) +chalcogenide (flake) Combination 6  6) chalcogenide (flake) +chalcogenide (non-flake) Combination 7  7) chalcogenide (non-flake) +chalcogenide (non-flake) Combination 8  8) non-chalcogenide (flake) +non-chalcogenide (flake) Combination 9  9) non-chalcogenide (flake) +non-chalcogenide (non-flake) Combination 10 10) non-chalcogenide(non-flake) + non-chalcogenide (non-flake)

Additional Chalcogen

Any of the methods described herein may be further optimized by using,prior to, during, or after the solution deposition and/or heating of oneor more of the precursor layers, any combination of (1) any chalcogensource that can be solution-deposited, e.g. a Se or S nano- ormicron-sized powder mixed into the precursor layers or deposited as aseparate layer, (2) chalcogen (e.g., Se or S) evaporation, (3) an H₂Se(H₂S) atmosphere, (4) a chalcogen (e.g., Se or S) atmosphere, (5) an H₂atmosphere, (6) an organo-selenium atmosphere, e.g. diethylselenide oranother organo-metallic material, (7) another reducing atmosphere, e.g.CO, and a (8) heat treatment. The stoichiometric ratio of microflakes toextra chalcogen, given as Se/(Cu+In+Ga+Se) may be in the range of about0 to about 1000.

For example as shown in FIG. 26A, a layer 1808 containing elementalchalcogen particles 1807 over the precursor layer 1806. By way ofexample, and without loss of generality, the chalcogen particles may beparticles of selenium, sulfur or tellurium. As shown in FIG. 26B, heat1809 is applied to the precursor layer 1806 and the layer 1808containing the chalcogen particles to heat them to a temperaturesufficient to melt the chalcogen particles 1807 and to react thechalcogen particles 1807 with the group IB element and group IIIAelements in the precursor layer 1806. The reaction of the chalcogenparticles 1807 with the group IB and IIIA elements forms a compound film1810 of a group IB-IIIA-chalcogenide compound as shown in FIG. 26C.Optionally, the group IB-IIIA-chalcogenide compound is of the formCu_(z)In_(1-x)Ga_(x)Se_(2(1-y))S_(y), where 0<x<1, 0≦y≦1, and 0.5≦z≦1.5.

If the chalcogen particles 1807 melt at a relatively low temperature(e.g., 220° C. for Se, 120° C. for S) the chalcogen is already in aliquid state and makes good contact with the group IB and IIIAnanoparticles in the precursor layer 1806. If the precursor layer 1806and molten chalcogen are then heated sufficiently (e.g., at about 375°C.) the chalcogen reacts with the group IB and IIIA elements in theprecursor layer 1806 to form the desired IB-IIIA-chalcogenide materialin the compound film 1810. As one nonlimiting example, the precursorlayer is between about 10 nm and about 5000 nm thick. In otherembodiments, the precursor layer may be between about 4.0 to about 0.5microns thick.

There are a number of different techniques for forming the IB-IIIAprecursor layer 1806. For example, the precursor layer 1806 may beformed from a nanoparticulate film including nanoparticles containingthe desired group IB and IIIA elements. The nanoparticles may be amixture elemental nanoparticles, i.e., nanoparticles having only asingle atomic species. Alternatively, the nanoparticles may be binarynanoparticles, e.g., Cu—In, In—Ga, or Cu—Ga or ternary particles, suchas, but not limited to, Cu—In—Ga, or quaternary particles. Suchnanoparticles may be obtained, e.g., by ball milling a commerciallyavailable powder of the desired elemental, binary or ternary material.These nanoparticles may be between about 0.1 nanometer and about 500nanometers in size.

One of the advantages of the use of nanoparticle-based dispersions isthat it is possible to vary the concentration of the elements within thecompound film 1810 either by building the precursor layer in a sequenceof sub-layers or by directly varying the relative concentrations in theprecursor layer 1806. The relative elemental concentration of thenanoparticles that make up the ink for each sub-layer may be varied.Thus, for example, the concentration of gallium within the absorberlayer may be varied as a function of depth within the absorber layer.

The layer 1808 containing the chalcogen particles 1807 may be disposedover the nanoparticulate film and the nanoparticulate film (or one ormore of its constituent sub-layers) may be subsequently sintered inconjunction with heating the chalcogen particles 1807. Alternatively,the nanoparticulate film may be heated to form the precursor layer 1806before disposing the layer 1808 containing elemental chalcogen particles1807 over precursor layer 1806. Additional disclosure on depositingchalcogen material may be found in co-pending U.S. patent applicationSer. No. 11/361,522 filed Feb. 23, 2006 and fully incorporated herein byreference for all purposes.

Referring now to FIG. 27A, it should be understood that any of theforegoing may also be used in a chalcogen vapor environment. It shouldbe understood that this may be used in a one stage process, a two stageprocess, or a multi-stage process. In two stage and/or multi-stageprocess, different atmospheres may optionally be used for each stage andsome may be inert atmospheres as described previously.

In another embodiment for use with a particle and/or microflakeprecursor material, it should be understood that overpressure fromchalcogen vapor is used to provide a chalcogen atmosphere to improveprocessing of the film and crystal growth. FIG. 27A shows a chamber 1050with a substrate 1052 having a contact layer 1054 and a precursor layer1056. Extra sources 1058 of chalcogen are included in the chamber andare brought to a temperature to generate chalcogen vapor as indicated bylines 1060. In one embodiment of the present invention, the chalcogenvapor is provided to have a partial pressure of the chalcogen present inthe atmosphere greater than or equal to the vapor pressure of chalcogenthat would be required to maintain a partial chalcogen pressure at theprocessing temperature and processing pressure to minimize loss ofchalcogen from the precursor layer, and if desired, provide theprecursor layer with additional chalcogen. The partial pressure isdetermined in part on the temperature that the chamber 1050 or theprecursor layer 1056 is at. It should also be understood that thechalcogen vapor is used in the chamber 1050 at a non-vacuum pressure. Inone embodiment, the pressure in the chamber is at about atmosphericpressure. Per the ideal gas law PV=nRT, it should be understood that thetemperature influences the vapor pressure. In one embodiment, thischalcogen vapor may be provided by using a partially or fully enclosedchamber with a chalcogen source 1062 therein or coupled to the chamber.In another embodiment using a more open chamber, the chalcogenatmosphere may be provided by supplying a source producing a chalcogenvapor. The chalcogen vapor may serve to help keep the chalcogen in thefilm or to provide the chalcogen to covert the precursor layer. Thus,the chalcogen vapor may or may not be used to provide excess chalcogen.In some embodiments, this may serve more to keep the chalcogen presentin the film than to provide more chalcogen into the film.

Optionally, this vapor or atmosphere maybe used as a chalcogen that isintroduced into an otherwise chalcogen free or selenium free precursorlayer. It should be understood that the exposure to chalcogen vapor mayoccur in a non-vacuum environment. The exposure to chalcogen vapor mayoccur at or near atmospheric pressure. These conditions may beapplicable to any of the embodiments described herein. The chalcogen maybe carried into the chamber by a carrier gas. The carrier gas may be aninert gas such as nitrogen, argon, or the like. This chalcogenatmosphere system may be adapted for use in a roll-to-roll system.

Referring now to FIG. 27B, it shown that the present invention may beadopted for use with a roll-to-roll system where the substrate 1070carrying the precursor layer may be flexible and configured as rolls1072 and 1074. The chamber 1076 may be at vacuum or non-vacuumpressures. The chamber 1076 may be designed to incorporate adifferential valve design to minimize the loss of chalcogen vapor at thechamber entry and chamber exit points of the roll-to-roll substrate1070.

Referring now to FIG. 27C, yet another embodiment of the presentinvention uses a chamber 1090 of sufficient size to hold the entiresubstrate, including any rolls 1072 or 1074 associated with using aroll-to-roll configuration.

While the invention has been described and illustrated with reference tocertain particular embodiments thereof, those skilled in the art willappreciate that various adaptations, changes, modifications,substitutions, deletions, or additions of procedures and protocols maybe made without departing from the spirit and scope of the invention.For example, with any of the above embodiments, particles of variousshapes and sizes may be used separately or in combination with oneanother. Although the examples provided herein describe microflakes, itshould be understood that flakes of other sizes may also be used in someembodiments of the invention. By way of nonlimiting example, microflakes(of solid group IIIA particles or particles of other compositions) maybe replaced by and/or mixed with nanoflakes wherein the lengths of theplanar nanoflakes are about 500 nm to about 1 nm. They may also be mixedwith spherical particles of the same or different composition. As anonlimiting example, the nanoflakes may have lengths and/or largestlateral dimension of about 300 nm to about 10 nm. In other embodiments,the nanoflakes may be of thickness in the range of about 200 nm to about20 nm. In another embodiment, these nanoflakes may be of thickness inthe range of about 100 nm to about 10 nm. In one embodiment, thesenanoflakes may be of thickness in the range of about 200 nm to about 20nm. As mentioned, some embodiments of the invention may include bothmicroflakes and nanoflakes. Other may include flakes that areexclusively in the size range of microflakes or the size range ofnanoflakes. With any of the above embodiments, the microflakes may bereplaced and/or combined with microrods which are substantially linear,elongate members. Still further embodiments may combine nanorods withmicroflakes in the precursor layer. The microrods may have lengthsbetween about 500 nm to about 1 nm. In another embodiment, the nanorodsmay have lengths between about 500 nm and 20 nm. In yet anotherembodiment, the nanorods may have lengths between about 300 nm and 30nm. Any of the above embodiments may be used on rigid substrate,flexible substrate, or a combinations of the two such as but not limitedto a flexible substrate that become rigid during processing due to itsmaterial properties. In one embodiment of the present invention, theparticles may be plates and/or discs and/or flakes and/or wires and/orrods of micro-sized proportions. In another embodiment of the presentinvention, the particles may be nanoplates and/or nanodiscs and/ornanoflakes and/or nanowires and/or nanorods of nano-sized proportions.Again, any of the foregoing may also be combined with sphericalparticles in a suspension. Some embodiments may have all sphericalparticles, all non-spherical particles, and/or mixtures of particles ofvarious shapes. It should be understood that the solid group IIIA-basedparticles may be used in single or multiple combination with particlesof other shapes and/or composition. This may include shapes such as butnot limited to spherical, planar, flake, platelet, other non-spherical,and/or single or multiple combinations of the foregoing. As formaterials, this may include alloys, elementals, chalcogenides,inter-metallics, solid-solutions and/or single or multiple combinationsof the foregoing in any shape or form. Use of solid particles withdispersions and/or emulsions of the foregoing is also envisioned. Thesolid solutions are described in pending U.S. patent application Ser.No. 10/474,259 and published as US20040219730, fully incorporated hereinby reference for all purposes. The following applications are also fullyincorporated herein by reference: Ser. Nos. 11/395,438, 11/395,668, and11/395,426 both filed Mar. 30, 2006. Any of the embodiments described inthose applications may be adapted for use with the solid IIIA-basedparticles described herein.

For any of the above embodiments, it should be understood that inaddition to the aforementioned, the temperature used during annealingmay also vary over different time periods of precursor layer processing.As a nonlimiting example, the heating may occur at a first temperatureover an initial processing time period and proceed to other temperaturesfor subsequent time periods of the processing. Optionally, the methodmay include intentionally creating one or more temperature dips so that,as a nonlimiting example, the method comprises heating, cooling,heating, and subsequent cooling. For any of the above embodiments, it isalso possible to have two or more elements of IB elements in thechalcogenide particle and/or the resulting film. Although thedescription herein uses an ink, it should be understood that in someembodiments, the ink may have the consistency of a paste or slurry.

Additionally, concentrations, amounts, and other numerical data may bepresented herein in a range format. It is to be understood that suchrange format is used merely for convenience and brevity and should beinterpreted flexibly to include not only the numerical values explicitlyrecited as the limits of the range, but also to include all theindividual numerical values or sub-ranges encompassed within that rangeas if each numerical value and sub-range is explicitly recited. Forexample, a size range of about 1 nm to about 200 nm should beinterpreted to include not only the explicitly recited limits of about 1nm and about 200 nm, but also to include individual sizes such as 2 nm,3 nm, 4 nm, and sub-ranges such as 10 nm to 50 nm, 20 nm to 100 nm, etc.. . .

For example, still other embodiments of the present invention may use aCu—In precursor material wherein Cu—In contributes less than about 50percent of both Cu and In found in the precursor material. The remainingamount is incorporated by elemental form or by non IB-IIIA alloys. Thus,a Cu₁₁In₉ may be used with elemental Cu, In, and Ga to form a resultingfilm. In another embodiment, instead of elemental Cu, In, and Ga, othermaterials such as Cu—Se, In—Se, and/or Ga—Se may be substituted assource of the group IB or IIIA material. Optionally, in anotherembodiment, the IB source may be any particle that contains Cu withoutbeing alloyed with In and Ga (Cu, Cu—Se). The IIIA source may be anyparticle that contains In without Cu (In—Se, In—Ga—Se) or any particlethat contains Ga without Cu (Ga, Ga—Se, or In—Ga—Se). Other embodimentsmay have these combinations of the IB material in a nitride or oxideform. Still other embodiments may have these combinations of the IIIAmaterial in a nitride or oxide form. The present invention may use anycombination of elements and/or selenides (binary, ternary, or multinary)may be used. Optionally, some other embodiments may use oxides such asIn₂O₃ to add the desired amounts of materials. It should be understoodfor any of the above embodiments that more than one solid solution maybe used, multi-phasic alloys, and/or more general alloys may also beused. For any of the above embodiments, the annealing process may alsoinvolve exposure of the compound film to a gas such as H₂, CO, N₂, Ar,H₂Se, Se vapor, S vapor, or other group VIA containing vapor. There maybe a two stage process where there is an initial anneal in a nongroup-VIA based atmosphere and then a second or more heating in groupVIA-based atmosphere. There may be a two stage process where there is aninitial anneal in a non group-VIA based atmosphere and then a secondheating in a non-group VIA based atmosphere, wherein VIA material isplaced directly on the stack for the second heating and additional isthe VIA-containing vapor is not used. Alternatively, some may use a onestage process to create a final film, or a multi-stage process whereeach heating step use a different atmosphere.

It should also be understood that several intermediate solid solutionsmay also be suitable for use according to the present invention. Asnonlimiting examples, a composition in the δ phase for Cu—In (about42.52 to about 44.3 wt % In) and/or a composition between the δ phasefor Cu—In and Cu₁₆In₉ may be suitable inter-metallic materials for usewith the present invention to form a group IB-IIIA-VIA compound. Itshould be understood that these inter-metallic materials may be mixedwith elemental or other materials such as Cu—Se, In—Se, and/or Ga—Se toprovide sources of the group IB or IIIA material to reach the desiredstoichiometric ratios in the final compound. Other nonlimiting examplesof inter-metallic material include compositions of Cu—Ga containing thefollowing phases: γ₁ (about 31.8 to about 39.8 wt % Ga), γ₂ (about 36.0to about 39.9 wt % Ga), γ₃ (about 39.7 to about −44.9 wt % Ga), thephase between γ₂ and γ₃, the phase between the terminal solid solutionand γ₁, and θ (about 66.7 to about 68.7 wt % Ga). For Cu—Ga, a suitablecomposition is also found in the range in between the terminalsolid-solution of and the intermediate solid-solution next to it.Advantageously, some of these inter-metallic materials may bemulti-phasic which are more likely to lead to brittle materials that canbe mechanically milled. Phase diagrams for the following materials maybe found in ASM Handbook, Volume 3 Alloy Phase Diagrams (1992) by ASMInternational and fully incorporated herein by reference for allpurposes. Some specific examples (fully incorporated herein byreference) may be found on pages 2-168, 2-170, 2-176, 2-178, 2-208,2-214, 2-257, and/or 2-259. It should also be understood that a particlemay have portions that are of a solid alloy and portions that are phaseseparated into individual elements or other alloys that are liquid.

It should be understood that any of the embodiments herein may beadapted for use in a one step process, or a two step process, or amulti-step process for forming a photovoltaic absorber layer. One stepprocesses do not require a second follow-up process to convert the filminto an absorber layer. A two step process typically creates a film thatuses a second process to convert the film into an absorber layer.Additionally, some embodiments may have anywhere from about 0 to about 5wt % oxygen in the shell.

It should be understood that the solid IIIA particles as describedherein may be used with solids, solid solutions, intermetallics,nanoglobules, emulsions, nanoglobule, emulsion, or other types ofparticles.

The publications discussed or cited herein are provided solely for theirdisclosure prior to the filing date of the present application. Nothingherein is to be construed as an admission that the present invention isnot entitled to antedate such publication by virtue of prior invention.Further, the dates of publication provided may be different from theactual publication dates which may need to be independently confirmed.All publications mentioned herein are incorporated herein by referenceto disclose and describe the structures and/or methods in connectionwith which the publications are cited. The following relatedapplications are fully incorporated herein by reference for allpurposes: U.S. patent application Ser. No. 11/290,633 entitled“CHALCOGENIDE SOLAR CELLS” filed Nov. 29, 2005, U.S. patent applicationSer. No. 10/782,017, entitled “SOLUTION-BASED FABRICATION OFPHOTOVOLTAIC CELL” filed Feb. 19, 2004, U.S. patent application Ser. No.10/943,657, entitled “COATED NANOPARTICLES AND QUANTUM DOTS FORSOLUTION-BASED FABRICATION OF PHOTOVOLTAIC CELLS” filed Sep. 18, 2004,U.S. patent application Ser. No. 11/081,163, entitled “METALLICDISPERSION”, filed Mar. 16, 2005, and U.S. patent application Ser. No.10/943,685, entitled “FORMATION OF CIGS ABSORBER LAYERS ON FOILSUBSTRATES”, filed Sep. 18, 2004, Ser. No. 60/804,649 filed Jun. 13,2006, and Ser. No. 60/804,565 filed Jun. 12, 2006, the entiredisclosures of which are incorporated herein by reference. The followingapplications are also incorporated herein by reference for all purposes:Ser. No. 11/361,498 entitled “HIGH-THROUGHPUT PRINTING OF SEMICONDUCTORPRECURSOR LAYER FROM MICROFLAKE PARTICLES” filed Feb. 23, 2006 andcommonly-assigned, co-pending application Ser. No. 11/361,433 entitled“HIGH-THROUGHPUT PRINTING OF SEMICONDUCTOR PRECURSOR LAYER FROMNANOFLAKE PARTICLES” filed Feb. 23, 2006, Ser. No. 60/804,565 filed Jun.12, 2006, Ser. No. 60/804,566 filed Jun. 12, 2006, Ser. No. 60/804,567filed Jun. 12, 2006, Ser. No. 60/804,569 filed Jun. 12, 2006, Ser. No.60/804,649 filed Jun. 13, 2006, and Ser. No. 60/804,647 filed Jun. 13,2006.

While the above is a complete description of the preferred embodiment ofthe present invention, it is possible to use various alternatives,modifications and equivalents. Therefore, the scope of the presentinvention should be determined not with reference to the abovedescription but should, instead, be determined with reference to theappended claims, along with their full scope of equivalents. Anyfeature, whether preferred or not, may be combined with any otherfeature, whether preferred or not. In the claims that follow, theindefinite article “A”, or “An” refers to a quantity of one or more ofthe item following the article, except where expressly stated otherwise.The appended claims are not to be interpreted as includingmeans-plus-function limitations, unless such a limitation is explicitlyrecited in a given claim using the phrase “means for.”

1. A process of forming a Group IBIIIAVIA solar cell absorber on a base,comprising: forming a film on the base, wherein the film issubstantially metallic and comprises Cu and In; forming a separatorlayer over the film, wherein the separator layer substantially comprisesa Group VIA material; forming a source layer over the separator layer,wherein the source layer substantially comprises Ga and it issubstantially metallic; forming a cap layer over the source layer,wherein the cap layer substantially comprises Se; and reacting the film,the separator layer, the source layer and the cap layer with each otherat a reaction temperature to form the Group IBIIIAVIA absorber layer,the absorber layer including a surface region of less than or equal to300 nm thickness, wherein the Ga/(Ga+In) molar ratio within the surfaceregion is in the range of 0.1-0.3.
 2. The process of claim 1, whereinthe reaction temperature is in the range of 500-600.degree. C.
 3. Theprocess of claim 2, wherein the film, the separator layer, the sourcelayer and the cap layer are distinct layers with no substantialintermixing, and the step of reacting includes rapidly heating the film,the separator layer, the source layer and the cap layer to the reactiontemperature with a heating rate of at least 50° C. per second. 4.(canceled)
 5. The process of claim 2, wherein the film further includesGa.
 6. The process of claim 5, wherein the film is in the form of amaterial stack made of distinct layers and the step of forming the filmcomprises: depositing a Cu layer over the base; and depositing at leasttwo additional layers including Ga and In over the Cu layer, wherein theGa/(Ga+In) ratio within the film is in the range of 0-0.7.
 7. Theprocess of claim 6, wherein the steps of depositing the Cu layer anddepositing at least two additional layers are carried out byelectroplating.
 8. The process of claim 7, wherein the step of formingthe separator layer comprises electroplating.
 9. The process of claim 8,wherein the step of forming the source layer comprises vapor deposition.10. The process of claim 9, wherein the step of forming the cap layercomprises vapor deposition.
 11. (canceled)
 12. The process of claim 11,wherein the fourth additive material includes at least one dopantmaterial selected from the group of Na, K and Li.
 13. The process ofclaim 12, wherein the molar ratio of Cu/In+Ga in the film is in therange of 0.8-1.0
 14. The process of claim 2, wherein the separator layerfurther includes a first additive material, wherein a ratio of the molaramount of the Group VIA material to the total molar amounts of the GroupVIA material and the first additive material is in the range of0.95-1.0.
 15. The process of claim 14, wherein the Group VIA materialincludes at least one of Se and Te, and the first additive materialincludes at least one of Cu, In, Ga and a dopant material selected fromthe group of Na, K and Li.
 16. (canceled)
 17. The process of claim 16,wherein the second additive material includes at least one of Cu, In,and a dopant material selected from the group of Na, K and Li. 18.(canceled)
 19. The process of claim 18, wherein the third additivematerial includes at least one of Te and a dopant material selected fromthe group of Na, K and Li.
 20. The process of claim 2, wherein at leastone of the separator layer, the source layer and the cap layer includesa dopant film comprising one of Na, K and Li.
 21. (canceled) 22.(canceled)
 23. The process of claim 1, wherein the step of forming theseparator layer comprises electroplating.
 24. A multilayer structure toform Group IBIIIAVIA absorber layers for solar cells, comprising: abase; a substantially metallic film formed over the base, wherein thesubstantially metallic film includes Cu and In; a separator layer formedover the substantially metallic film, the separator layer substantiallycomprises a Group VIA material; a substantially metallic source layerformed over the separator layer, the source layer substantiallycomprises Ga; and a cap layer formed over the source layer, the caplayer substantially comprises Se.
 25. (canceled)
 26. (canceled)
 27. Themultilayer structure of claim 26, wherein the Group VIA materialcomprises at least one of Se and Te, and the first additive materialcomprises at least one of Cu, In, Ga, and a dopant material selectedfrom the group of Na, K and Li. 28-32. (canceled)
 33. The multilayerstructure of claim 32, wherein the substrate comprises one of glasssheet, polyimide foil, stainless steel foil and aluminum alloy foil, andthe contact layer comprises Mo. 34-39. (canceled)